Support m4 image flash by fastboot: 'fastboot flash m4_os <path-to-image>'.

For 7ulp, m4 image will be excuted on POR and it doesn't depand on A
core, so the m4 image will be flashed to nor flash using 'sf' command.
For imx8m, m4 image will be loaded and excuted by A core. The m4 image
will be flashed to physical 'm4_os' partition and can be excuted by
'bootmcu' command.

Change-Id: I43bf0c9a484c5633cfe32dde3d738e33afdc775b
Signed-off-by: Luo Ji <ji.luo@nxp.com>