arm: fix arm kernel boot for non zero start addr

arm: fix arm kernel boot for non zero start addr

Booting an arm kernel has been broken a while when booting from non zero start
address. This is due to the order of events: board init loads the kernel and
sets register 15 to the start address and then qemu_system_reset reset the cpu
making register 15 zero again.

This patch fixes the usage of the register 15 start address trick in
combination with arm_load_kernel.

Signed-off-by: Lars Munch <lars@segv.dk>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>