gblsigntool: Import from aosp/gbl-mainline

Changes since last import:
1. aosp/3699107
2. aosp/3699126

Test: atest --host gblsigntool_test
Change-Id: Ica89c98abf343fa304303868f058dc48b6ba404f
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>