makefiles/tools/serial: ensure PORT is set and fail early.

[fjmolinas]

Contribution description

This commit has been cherry-picked from #10342

By ensuring the PORT auto-detection worked, we can give meaningful
error messages and fail earlier.

This uses ensure_value from makefiles/utils/checks.mk. An include was
added to Makefile.include to make this fuction available to all other
makefiles.

This also avoids evaluating $(PORT).

Testing procedure

Set PORT_LINUX empty:

PORT_LINUX='' BOARD=cc2538dk make -C examples/hello-world/ term

make: Entering directory '/home/francisco/workspace/RIOT/examples/hello-world'
/home/francisco/workspace/RIOT/examples/hello-world/../../Makefile.include:646: *** No port set.  Stop.
make: Leaving directory '/home/francisco/workspace/RIOT/examples/hello-world'

Issues/PRs references

From #10342, closes #10342

[aabadie]

I'm not sure I understand the goal of this PR.

if I do

PORT_LINUX='' BOARD=cc2538dk make -C examples/hello-world/ term

it indeed fails early. But this is not the usual way of setting a port. The variable used in general is PORT. And if I set PORT to an empty value, then it doesn't fail early:

PORT='' BOARD=cc2538dk make -C examples/hello-world/ term
make: Entering directory '/work/riot/RIOT/examples/hello-world'
/work/riot/RIOT/dist/tools/pyterm/pyterm -p "" -b "115200" 
Twisted not available, please install it if you want to use pyterm's JSON capabilities
No port specified, using default (/dev/ttyUSB0)!
2019-10-17 10:26:23,468 # Connect to serial port /dev/ttyUSB0
2019-10-17 10:26:23,468 # Cannot connect to serial port /dev/ttyUSB0: could not open port /dev/ttyUSB0: [Errno 2] No such file or directory: '/dev/ttyUSB0'
make: *** [/work/riot/RIOT/examples/hello-world/../../Makefile.include:647: term] Error 1
make: Leaving directory '/work/riot/RIOT/examples/hello-world'

[aabadie]

Why not just trying to merge #10342 ? (what @smlng is silently trying to do).

[fjmolinas]

Why not just trying to merge #10342 ? (what @smlng is silently trying to do).

I-m fine with that.

[fjmolinas]

I-m fine with that.

It was just to get the commit in faster since @jcarrano is not working on the project anymore.

[aabadie]

ACK and go

[fjmolinas]

@aabadie thanks for the review,
@jcarrano thanks for doing the work :)