scripts: ci: check_compliance: add BOARD_REVISION in Kconfig.board.v2

This is required if boards make use of such definition in their Kconfig
files. In Kconfig.board.v2, only `boards/Kconfig.v2` is loaded, but
BOARD_REVISION is part of `boards/Kconfig`, which can't be loaded in this
context.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

scripts/ci/Kconfig.board.v2

@@ -6,5 +6,8 @@
 
 mainmenu "Zephyr board / SoC v2 Configuration"
 
+config BOARD_REVISION
+	def_string "$(BOARD_REVISION)"
+
 source "boards/Kconfig.v2"
 source "soc/Kconfig.v2"