arm: relocate_code(): Use __image_copy_end for end of relocation

Use __image_copy_end instead of __bss_start for the end of the image to
relocate. This is the same as commit 033ca724, but applied to all ARM start.S.

This is a more appropriate symbol naming for an image copy & relocate feature,
and this also saves a useless copy of data put between __image_copy_end and
__bss_start in linker scripts (e.g. relocation information, or MMU
initialization tables used only before jumping to the relocated image).

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>