Implement "take a screenshot" feature

* Like in android - press power+volume down, screenshots are saved
  in /sdcard/Pictures/Screenshots (if /sdcard is mounted) or /tmp

Change-Id: Iaefa15b11a1d5fdfac57d77388db1621f378a8d4
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Remove unused twbootloader.cpp

Change-Id: I5a1d52d69662a97049fad18dc760e205c193f381
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
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