Do not wipe block misc on bootloader msg update

When set_bootloader_message_block() is called, it fopens /misc in write
binary mode, wiping all contents other than what is being written. The
bootloader msg structure is only 1024+32+32 bytes, so some manufacturers
store more than just a bootloader msg on /misc. fopen in read+append
mode so that only the bootloader msg is written and the rest of the
partition is left untouched.

Change-Id: I2d2fbdf067282744864a19d404ca7dc12f688a98