[POWERPC] Move bootwrapper ELF parsing routines to a file

The ELF parsing routines local to arch/powerpc/boot/main.c are useful
to other callers therefore move them to their own file.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>