gsc: add env var support

Migrate over to using the env var format for specifying gsc keys.

BUG=b:400968066
BRANCH=None
TEST=None

Change-Id: I6e296701c8fd1b12795dd8db054bab1f04dbc1df
Signed-off-by: Benjamin Shai <bshai@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6330327
Reviewed-by: George Engelbrecht <engeg@google.com>
Reviewed-by: Vadim Bendebury <vbendeb@google.com>
3 files changed