Nokia RX-51: Add support for booting kernel in zImage format

Nokia RX-51: Add support for booting kernel in zImage format

Enable U-Boot bootz command and update env scripts to try loading also
zImage file and to try booting via bootz command.

Update also lowlevel_init.S code for checking validity of zImage magic to
correctly relocate kernel in zImage format.

This change allows U-Boot to directly boot Linux kernel without need for
converting kernel image into U-Boot uImage format.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210618132704.32066-1-pali@kernel.org