tools: imx8mimage: Restore the original __ALIGN_MASK() macro

Since commit c738adb8

 ("tool: Move ALIGN_MASK to header as common MACRO")
the i.MX8MQ EVK board no longer boots.

The reason is that imx8mimage.c used a custom __ALIGN_MASK() macro, so
restore the original macro to fix the boot and rename it accordingly.

Reported-by: Lukas Rusak <lorusak@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Ye Li <ye.li@nxp.com>