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

doc/README.nokia_rx51

@@ -11,13 +11,14 @@ a kernel. In order to transparently boot the original kernel, it will be
 appended to u-boot.bin at 0x40000. NOLO will load the entire image into
 (random) memory and execute u-boot, which saves hw revision, boot reason
 and boot mode ATAGs set by NOLO. Then the bootscripts will attempt to load
-uImage or boot.scr from a fat, ext2/ext3 or ext4 filesystem in external
+uImage, zImage or boot.scr from a fat or ext2/3/4 filesystem on external
 SD card or internal eMMC memory. If this fails or keyboard is closed then
 the appended kernel image will be booted using some generated and some
 stored ATAGs (see boot order).
 
-For generating combined image of u-boot and kernel there is a simple script
-called u-boot-gen-combined. It is available in following repository:
+For generating combined image of u-boot and kernel (either in uImage or zImage
+format) there is a simple script called u-boot-gen-combined. It is available in
+following repository:
 
   https://github.com/pali/u-boot-maemo
 
@@ -41,7 +42,8 @@ Boot from SD or eMMC in this order:
  * 1.
    * 1.1 find boot.scr on first fat partition
    * 1.2 find uImage on first fat partition
-   * 1.3 same order for 2. - 4. fat partition
+   * 1.3 find zImage on first fat partition
+   * 1.4 same order for 2. - 4. fat partition
  * 2. same as 1. but for ext2/3 partition
  * 3. same as 1. but for ext4 partition