recovery: block writes to eMMC partitions when managing SD install

Legacy OTA ROM updates attempt to format and/or mount the eMMC system
and boot partitions directly, instead of using the new
install-location-agnostic symlinks.  Users attempting to flash one of
these old OTAs using a recovery installed to SD card would therefore end
up modifying their eMMC, a potentially disastrous result.

Prevent this from happening by bind mounting /dev/null over the eMMC
system, boot, and userdata partition devices when recovery is managing
an SD card install.  This doesn't cause the OTA update to fail (it'll
dump all of its files to the rootfs, potentially causing an
out-of-memory situation), but it does prevent unexpected modification of
the eMMC.

If you actually need to work with the eMMC partitions from a SD card
recovery, run "/sbin/recovery_emmc_protect.sh stop" to disable this
protection.

Change-Id: Ie1d01ef869a217dbb2d0a9f0eefc4d06f97f15fd
recovery: fix reboot-to-recovery from within recovery

Fix two problems preventing reboot-to-recovery from working in recovery:

* /rom wasn't necessarily mounted before trying to write to /rom/bcb
* /system/bin was empty, so the commands in the shell script fragment
  weren't found

Change-Id: If30a87caa62190804e78ca76370856d427e0ea5a