Temporary hack for making T->S downgrade work

Downgrade OTA was broken because generic_ramdisk was removed from boot
partition in android T. Neither does O6 have init_boot. O6 also doesn't
include generic ramdisk in vendor boot, so no way for otatools to locate
contents of generic ramdisk. As a hack, omit boot partition timestamp if
we can't find ramdisk.

Test: generate T->S downgrade OTA for O6
Bug: 231656318
Change-Id: I9f5359462332aadadc324348873a3a4b2b126068