fix: list supported boards when BOARD is missing

[giri256]

Fixes #392

Commit 2d68ec5 moved board-specific configuration earlier in the Makefile and removed the existing board validation block. As a result, running bare make reaches the MCU selector with an empty MCU_SUB_VARIANT and reports Sub Variant is unknown instead of showing the supported boards.

This restores validation before board.mk is loaded. The supported list is derived from actual src/boards/*/board.mk files, so it stays current as boards are added or removed. Missing and invalid BOARD values now both produce actionable output and preserve a nonzero exit.

Verification with GNU Make 4.3:

• bare make lists all 49 supported boards and exits with BOARD not defined
• make BOARD=not_a_board lists the same choices and exits with Invalid BOARD specified
• make -n BOARD=feather_nrf52840_express clean
• make -n BOARD=feather_nrf52832 clean