Allow custom bootloader msg offset in block misc

Use board define BOARD_RECOVERY_BLDRMSG_OFFSET with a decimal integer
to define a custom offset where the bootloader message should be
read/written.

Edify commands get_stage and set_stage need to be aware of the
custom bootloader msg offset because they write the stage directly
to the BCB.

Change-Id: Id13a23dd41bb7d907b96d657b8e21eb839dfeaa9