bootloader_message: Allow TWRP to ignore wipe-data command into misc

 * Rebase of https://gerrit.omnirom.org/#/c/20750/ patch-set 3
 * Original commit message:

fix: recently a change in AOSP causes a bootloop to TWRP
(and therefore all ROM's based on it)
Reason: AOSP enables use of /misc partition and normally the fstab in
TWRP will not have /misc added. The problem is now when Android writes
a wipe command to /misc it will force TWRP to boot but TWRP will do
nothing - leaving the cmd in /misc. This results in a bootloop to TWRP.
Solution: I added a new var TW_IGNORE_MISC_WIPE_DATA which can be
set to "true" in Boardconfig.mk of a device requiring this which then
ignores /misc commands like it would be when /misc would be just not in
fstab BUT with this change TWRP clears the bit in /misc so a normal boot
can happen.
Besides that if the device do not have an own recovery key combo and
/misc is enabled we want to open TWRP after walking through the factory
reset screen - without actually doing a factory reset.
kudos to nailyk (and DevUt) for the help with the C++ code!

Change-Id: If6fed5872015f1a66304e3edbdb787c507236f69