arm: fix incorrect monitor protection region in FLASH

Monitor protection region in FLASH did not cover .rel.dyn
and .dynsym sections, because it uses __bss_start to compute
monitor_flash_len. Use _end instead.

Add _end to linker scripts for end of u-boot image
Add _end_ofs to all the start.S.

Signed-off-by: Po-Yu Chuang <ratbert@faraday-tech.com>