add option to generate two-step recovery files

When run with the -2 option, ota_from_target_files will generate a
package (full or incremental) that does some extra reboots in order to
install the new recovery first, so that the rest of the installation
is done with the new recovery.  This can be useful if (say) the
package installation needs some features from the newer kernel.

For incremental packages, the verification phase is still done with
the old recovery.

This is only supported on devices where the misc partition is EMMC
(not MTD).

Two-step packages are slower to install and possibly confusing to
users (they will see their device reboot four times instead of twice),
so only use this option if necessary.

Change-Id: I3267d905e5e8eb1a1eb61bf48255b8b24ffc4ad1