sparc,sparc64: unify boot/

Simple unification:
o renamed piggyback to *_32.c/*_64.c
o copied content of Makefile from sparc64 to sparc and guard it
o updated sparc/boot/.gitignore
o deleted remaining files in sparc64/boot

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>