Allow custom bootloader msg offset in block misc

Globally define BOARD_RECOVERY_BLDRMSG_OFFSET with a decimal integer
to offset the read/write location in misc where the bootloader message
should appear. Example:

  BOARD_GLOBAL_CFLAGS := -DBOARD_RECOVERY_BLDRMSG_OFFSET=2048

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: I6f3de1c2e3745c5535c3b95399abb8b2e2d01446
(cherry picked from commit 171e4bd8)