mkconfig: include board config.h before asm/config.h

swapping the include order suppresses warnings for board configs
that define their own CONFIG_MAX_MEM_MAPPED:

In file included from /home/r1aaha/git/u-boot/include/config.h:5,
                from /home/r1aaha/git/u-boot/include/common.h:35,
                from simpc8313.c:26:
/home/r1aaha/git/u-boot/include/configs/SIMPC8313.h:81:1: warning:
"CONFIG_MAX_MEM_MAPPED" redefined
In file included from /home/r1aaha/git/u-boot/include/config.h:4,
                from /home/r1aaha/git/u-boot/include/common.h:35,
                from simpc8313.c:26:
/home/r1aaha/git/u-boot/include/asm/config.h:28:1: warning: this is
the location of the previous definition

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>