rootdir: record last build signature and kernel version

(cherry pick from commit 93394034)

Rotate /default.prop and /proc/version into /data/misc/recovery/
as an aid in determining the vintage of the LAST_LOGCAT and
LAST_DMESG in the bugreport collection.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Test: manually confirm content rotation through reboots
Bug: 62793047
Change-Id: Ibbe546c76041f20e308e58e5548939afac75db97