bootmem: Separate out CONFIG_NO_BOOTMEM code into nobootmem.c
mm/bootmem.c contained code paths for both bootmem and no bootmem
configurations. They implement about the same set of APIs in
different ways and as a result bootmem.c contains massive amount of
#ifdef CONFIG_NO_BOOTMEM.
Separate out CONFIG_NO_BOOTMEM code into mm/nobootmem.c. As the
common part is relatively small, duplicate them in nobootmem.c instead
of creating a common file or ifdef'ing in bootmem.c.