BoardConfigCommon: set the bootloader msg offset

HTC offsets the bootloader message to 0x800 (2048) in /misc. This
prevents overwriting other stored variables in misc, such as the most
recently installed stock OS version.

see here https://github.com/CyanogenMod/android_device_htc_s4-common/commit/384b1780dd221a160bab18be2d939411bcc40057