Fix BOARD_RECOVERY_BLDRMSG_OFFSET CFLAG

Including BOARD_RECOVERY_BLDRMSG_OFFSET in BOARD_RECOVERY_DEFINES turns
the define into a string. We need BOARD_RECOVERY_BLDRMSG_OFFSET to be an
int.

Change-Id: Ib75dec0123e6fb318c4c8192645485683c6a5075