allow recovery packages to wipe cache

updater now has a function "wipe_cache();" which causes recovery to
wipe the cache partition after the successful installation of the
package.  Move log copying around a bit so logs and the last_install
flag file are copied to cache after it's wiped.

Bug: 5314244
Change-Id: Id35a9eb6dcd626c8f3a3a0076074f462ed3d44bd