sunxi: spl: Detect at runtime where SPL was read from

sunxi: spl: Detect at runtime where SPL was read from

Make possible using a single `u-boot-sunxi-with-spl.bin` binary for both NAND
memory and SD card. Detection where SPL was read from is implemented in
`spl_boot_device`.

Signed-off-by: Daniel Kochmański <dkochmanski@turtle-solutions.eu>
CC: Roy Spliet <r.spliet@ultimaker.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
[hdegoede@redhat.com: Some small coding style fixes]
Acked-by: Hans De Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>