rootdir: record last build fingerprint

We can't copy /default.prop so just write the value of
$(ro.build.fingerprint) to /data/misc/recovery/ro.build.fingerprint
and rotate it after reboot instead.

Bug: 62793047
Test: manual - reboot phone and check /data/misc/recovery
Change-Id: I130a4b7a01d9e1bfe9baecde2781626eb72e768b
(cherry picked from commit b9b76de6)