Make package_extract_file() take an optional third argument which is
the pathname (in the package zip) of a map of don't-care regions to
skip over when writing the file.
Modify syspatch() to take source and target don't-care maps and use
them when patching the system partition.
Add the wipe_block_device() function to do a discard of all data on
the partition.
Change-Id: I8c856054edfb6aab2f3e5177f16d9d78add20be4
c9d6e4ff51