boot: script: The boot.scr file for K+P's boards

By using this file one can avoid cluttering <board>.h file with u-boot
HUSH commands necessary for booting target device.

With such approach the commands are stored only in one place and can be
reused if needed.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>