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.

Change-Id: Id13a23dd41bb7d907b96d657b8e21eb839dfeaa9
Only disable recovery flash if it is truly "stock" recovery

 Some roms need the recovery-from-boot.p for auto update
 of recovery. Typically the use install-[romname]-recovery.sh.
 So only disable if install-recovery.sh is present.

Change-Id: I01f89afc87b9d375d67b44b8b5e9228868a47d69