sparc: fix tftpboot.img build

Kjetil Oftedal mentioned that piggyback_32 was failing
when building a sparc image.

I tracked this down to the fact that the kernel no longer
provided an absolute symbol named "end".

Commit 86ed40bd6fe511d26bb8f3fa65a84cb65c235366 ("sparc: unify sections.h")
renamed end to _end but failed to update piggyback_32.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Kjetil Oftedal <oftedal@gmail.com>
Cc: Robert Reif <reif@earthlink.net>
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>