Specify write-offset for bootloader msg in /misc

HTC stores more than just a bootloader message in /misc. The bootloader
message is offset to byte 0x800 (2048), verified by choosing "factory
reset" from hboot, finding where "recovery --wipe_data" was written to
/misc and then backing up 64bytes, in accordance with struct
bootloader_message.

Requires the following commits to recovery:
 Do not wipe block misc on bootloader msg update
 I2d2fbdf067282744864a19d404ca7dc12f688a98

 Allow custom bootloader msg offset in block misc
 Id13a23dd41bb7d907b96d657b8e21eb839dfeaa9

Change-Id: I1b088f1819ae27cc48ec571047d0f851a68eb425