mkconfig: Create board directory (CONFIG_BOARDDIR) in include/config.h

This patch extends the mkconfig script to automatically create a define
for the board directory in include/config.h:

#define CONFIG_BOARDDIR board/amcc/canyonlands

This is needed for the upcoming PPC4xx linker script consolidation,
where the PPC440 platforms need to include a board specific file in
the common linker script.

Signed-off-by: Stefan Roese <sr@denx.de>