MA-17159 Implement bootloader menu for imx7ulp

MA-17159 Implement bootloader menu for imx7ulp

According to the spec of android wear, the device MUST provides a
bootloader menu for debugging purpose.

This commit implements a simple bootloader menu based on imx7ulp_evk
revb board, the menu will show when booting with 'VOL+' key pressed,
users can press "VOL+" to choose the item, then press "ONOFF" key to
confirm.

Test: bootloader menu show on imx7ulp_evk revb.

Change-Id: I80638a43afa17e312e633b05888c62440380b42b
Signed-off-by: Ji Luo <ji.luo@nxp.com>
(cherry picked from commit 9d5995caf7146d4955fba2524a7f25d761049e28)