cmd: avb: free partition buffer upon verify completion

Doing the same as the unittests for libavb [1].

Allows to run 'avb verify' multiple times which can be useful after a
failure to be able to re-flash the partition and try again.

[1]
https://android.googlesource.com/platform/external/avb/+/refs/tags/android-9.0.0_r37/test/avb_slot_verify_unittest.cc#156



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>