[POWERPC] bootwrapper: Allow platforms to call library zImage_start

Some platforms might need to run some code before the zImage start, but
could otherwise use the bss clear and relocation code.   Export the
start address strongly as zImage_start_lib.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>