Add mkbootimg_test

Add host-side unit test for mkbootimg / unpack_bootimg.
Right now it only have one testcase that tests vendor_boot version 4.
Other testcases should be added when other parts of the mkbootimg tool
are touched or refactored.

Bug: 162864255
Test: atest --host mkbootimg_test
Change-Id: Ibe21e8082b60f34fb36a8ba86716f9c05fc888a3