Backport "system-imgage-upgrader: Fix invalid argument to verify_signature"

* commit 83f07bc84cf280f871e75343fc2533364aad961d
  Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

Change-Id: I6dd14aaa97a9bdd33bf460ac70be503b074d4139
Backport "system-image-upgrade: Pass --ignore-time-conflict to gpg"

* commit 3fa5e3a53ca2c34a74291cef7a9f936488936c6a

  We need to support devices with invalid time as they tend to reset to
  the unix epoch when out of battery.

  Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

Change-Id: Icb35282622fbceb23caa4b1deb213dd4b654155a
Backport "system-image-uprader: Fix GPG validation"

* commit 3cf886e620909c06efdcef3b4a9d61f53691f634
  Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

Change-Id: Idabf90b4fab882b4507320d2c45a23d153bff39b