Kconfig: Remind users to delete .config when switching boards

Because Kconfig default values *ONLY* get set when they are first
configured, if you switch mainboards with an existing .config,
the values will not be set as expected for the new board.

This seems to confuse most users, so put a warning in a visible
location to let them know.

Change-Id: Ie6a9c2d139ecd841d654943f14c119ebafd632f2
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/17939
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/mainboard/Kconfig

@@ -1,3 +1,5 @@
+comment "Important: Run 'make distclean' before switching boards"
+
 choice
 	prompt "Mainboard vendor"
 	default VENDOR_EMULATION