mkconfig: split the board make target to multiple config targets

To simplify the top level makefile it useful to be able to parse
the top level makefile target to multiple individual target, then
put them to the config.h, leave the board config file to handle
the different targets.

Note that this method uses the '_'(underline) as the delimiter when
splits the board make target.

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>

This also reverts commit 511c02f6.

Signed-off-by: Wolfgang Denk <wd@denx.de>