A/B switch logic will be moved to SPL stage if dual bootloader
feature is enable, in such case, we just need to verify single
slot which is selected in SPL stage.

Test: verify and boot ok for imx8m.

Change-Id: Iafe0d2d4aea1c178551940808416eec4a3547259
Signed-off-by: Luo Ji <ji.luo@nxp.com>