fix build error; also add /proc/config support

arch/mips/built-in.o: In function `prom_putchar':
(.text.prom_putchar+0x0): multiple definition of `prom_putchar'
arch/mips/ralink/built-in.o:(.text.prom_putchar+0x0): first defined here
Makefile:829: recipe for target 'vmlinux' failed
make[5]: *** [vmlinux] Error 1