[build] Add STM32 and SAM support to ELF2UF2 tool

src/modm/board/feather_m4/module.md

@@ -65,12 +65,16 @@ The Feather M4 has a [UF2](https://github.com/microsoft/uf2) bootloader. By
 using a utility to convert the '.elf' files (generated by scons or make) to
 '.uf2' files, it is then possible to copy the uf2 file directly to the board.
 
-One of the more common tools for this conversion is ['uf2conv.py'](https://github.com/microsoft/uf2/tree/master/utils) (although the elf file needs to be converted to a
-binary or hex file first). The converter that is supplied with modm, 'elf2uf2.py',
-is ONLY for the rp2040, and will not work here.
-
-However, the bootloader on the Feather is compatible with [BOSSA](https://www.shumatech.com/web/products/bossa); so the easiest method when working with modm is to use
-'scons program-bossac' or 'make program-bossac' to load your program into
-flash. (Be sure to press the reset button twice to get the board into bootloader
-mode before flashing.)
+In modm, the conversion utility is called 'elf2uf2.py' (in tools/modm_tools),
+and is incorporated into the build system. Use 'scons uf2' or 'make uf2' to
+create the uf2 file.
+
+Alternatively, the bootloader on the Feather is compatible with [BOSSA](https://www.shumatech.com/web/products/bossa);
+so another method is to use 'scons program-bossac' or 'make program-bossac' to
+load your program into flash.
+
+With either method, you must press the reset button twice to get the board into
+bootloader mode before flashing with BOSSA or UF2 copying. (In bootloader mode,
+a volume named FEATHERBOOT will be mounted; and that is where the UF2 file
+should be copied.)