make: remove use of export with LINKFLAGS variable

[aabadie]

Contribution description

This PR removes the remaining unnecessary uses of export along with the LINKFLAGS variable.

Testing procedure

• A green Murdock
• Verify that firmwares are unchanged on the related platforms:

STM32

This PR:

$ RIOT_VERSION=test make BOARD=nucleo-l073rz -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   8356	    120	   2568	  11044	   2b24	/work/riot/RIOT/examples/hello-world/bin/nucleo-l073rz/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=nucleo-l073rz -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   8356	    120	   2568	  11044	   2b24	/work/riot/RIOT/examples/hello-world/bin/nucleo-l073rz/hello-world.elf

SAM

This PR:

$ RIOT_VERSION=test make BOARD=samr21-xpro -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   8488	    120	   2560	  11168	   2ba0	/work/riot/RIOT/examples/hello-world/bin/samr21-xpro/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=samr21-xpro -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   8488	    120	   2560	  11168	   2ba0	/work/riot/RIOT/examples/hello-world/bin/samr21-xpro/hello-world.elf

RISCV

This PR:

$ RIOT_VERSION=test make BOARD=hifive1b -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  10580	    112	   2408	  13100	   332c	/work/riot/RIOT/examples/hello-world/bin/hifive1b/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=hifive1b -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  10580	    112	   2408	  13100	   332c	/work/riot/RIOT/examples/hello-world/bin/hifive1b/hello-world.elf

MIPS

This PR:

$ RIOT_VERSION=test make BOARD=pic32-clicker -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  71284	   2412	   6008	  79704	  13758	/work/riot/RIOT/examples/hello-world/bin/pic32-clicker/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=pic32-clicker -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  71284	   2412	   6008	  79704	  13758	/work/riot/RIOT/examples/hello-world/bin/pic32-clicker/hello-world.elf

MSP430

This PR:

$ BUILD_IN_DOCKER=1 RIOT_VERSION=test make BOARD=z1 -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   4358	      6	    984	   5348	   14e4	/data/riotbuild/riotbase/examples/hello-world/bin/z1/hello-world.elf

master:

$ BUILD_IN_DOCKER=1 RIOT_VERSION=test make BOARD=z1 -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
   4358	      6	    984	   5348	   14e4	/data/riotbuild/riotbase/examples/hello-world/bin/z1/hello-world.elf

native

This PR:

$ RIOT_VERSION=test make BOARD=native -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  24784	    576	  47728	  73088	  11d80	/work/riot/RIOT/examples/hello-world/bin/native/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=native -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  24784	    576	  47728	  73088	  11d80	/work/riot/RIOT/examples/hello-world/bin/native/hello-world.elf

ARMv7

This PR:

$ RIOT_VERSION=test make BOARD=msba2 -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  11416	    124	   6936	  18476	   482c	/work/riot/RIOT/examples/hello-world/bin/msba2/hello-world.elf

master:

$ RIOT_VERSION=test make BOARD=msba2 -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  11416	    124	   6936	  18476	   482c	/work/riot/RIOT/examples/hello-world/bin/msba2/hello-world.elf

• Verify that libfixmath-unittests is unchanged (only on native here):

This PR:

$ RIOT_VERSION=test make BOARD=native -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  87200	    608	  47728	 135536	  21170	/work/riot/RIOT/tests/pkg_libfixmath_unittests/bin/native/tests_pkg_libfixmath_unittests.elf

master:

$ RIOT_VERSION=test make BOARD=native -C examples/hello-world clean all
[...]
   text	   data	    bss	    dec	    hex	filename
  87200	    608	  47728	 135536	  21170	/work/riot/RIOT/tests/pkg_libfixmath_unittests/bin/native/tests_pkg_libfixmath_unittests.elf

• Verify that printf-float unittest is unchanged (only on native here):

This PR:

$ RIOT_VERSION=test make -C tests/unittests/ tests-printf_float
[...]
   text	   data	    bss	    dec	    hex	filename
  28300	    652	  47752	  76704	  12ba0	/work/riot/RIOT/tests/unittests/bin/native/tests_unittests.elf

master:

$ RIOT_VERSION=test make -C tests/unittests/ tests-printf_float
[...]
   text	   data	    bss	    dec	    hex	filename
  28300	    652	  47752	  76704	  12ba0	/work/riot/RIOT/tests/unittests/bin/native/tests_unittests.elf

Issues/PRs references

Ticks the LINKFLAGS item in #10850

[fjmolinas]

The export are indeed not needed, only used to assign to _LINK. Thanks for the good work @aabadie!

(NOTE: had to rebase this branch to test build diffs)

[fjmolinas]

ACK