MA-15158 Set spl recovery mode for dual bootloader

MA-15158 Set spl recovery mode for dual bootloader

The A/B slot selection is moved to spl, it may lead to hang
if no bootable slots found. The only way to recover the board
is re-flash images with uuu tool, which is quite inconvenient
for some customers who can't enter serial download mode.

This patch will set "spl recovery mode" which will give us a
chance to re-flash images with fastboot commands.

Test: Enter spl recovery mode and flash images when no bootable
      slots found.

Change-Id: I31278f5212bde7609fe2f49e77b3849e92c0c516
Signed-off-by: Ji Luo <ji.luo@nxp.com>