[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>

arch/powerpc/boot/crt0.S

@@ -19,6 +19,8 @@ _zimage_start_opd:
 	.weak	_zimage_start
 	.globl	_zimage_start
 _zimage_start:
+	.globl	_zimage_start_lib
+_zimage_start_lib:
 	/* Work out the offset between the address we were linked at
 	   and the address where we're running. */
 	bl	1f