u-boot: Add savedefconfig task

Add savedefconfig task which U-Boot supports (unfortunately not all
consumers of cml1 support this).

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>