Allow --disable-verification when top-level vbmeta is in 'boot'

If top-level vbmeta struct is in 'boot' partition,
the "flags" element offset of top-level vbmeta header
from the beginning of boot partition is 123 plus
top-level vbmeta offset recorded in the boot image footer.

Bug: 139639521
Test: m fastboot
Test: fastboot --disable-verification flashall
Test: fastboot --disable-verification flash boot boot.img, checks the
      flag isn't changed if the device has vbmeta partition
Test: fastboot --disable-verification flash vbmeta vbmeta.img
Change-Id: Ibf73c9330639e971ee3873ac19f072cf7baed55b