The current SPL header, created by the 'mksunxiboot' tool, has size
32 bytes. But the code in the boot ROM stores the information about
the boot media at the offset 0x28 before passing control to the SPL.
For example, when booting from the SD card, the magic number written
by the boot ROM is 0. And when booting from the SPI flash, the magic
number is 3. NAND and eMMC probably have their own special magic
numbers too.

Currently the corrupted byte is a part of one of the instructions in
the reset vectors table:

    b     reset
    ldr   pc, _undefined_instruction
    ldr   pc, _software_interrupt      <- Corruption happens here
    ldr   pc, _prefetch_abort
    ldr   pc, _data_abort
    ldr   pc, _not_used
    ldr   pc, _irq
    ldr   pc, _fiq

In practice this does not cause any visible problems, but it's still
better to fix it. As a bonus, the reported boot media type can be
later used in the 'spl_boot_device' function, but this is out of
the scope of this patch.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>