Add an option to input the boot variables for OTA package generation

The values of the ro.boot* variables are not part of the image files
and are provided (e.g. by bootloaders) at runtime. Meanwhile, their
values may affect some of the device build properties, as a different
build.prop file can be imported by init during runtime.

This cl adds an option to accepts a list of possible values for some
boot variables. The OTA generation script later use these values to
calculate the alternative runtime fingerprints of the device; and
list the device names and fingerprints in the OTA package's metadata.

The OTA metadata is verified by the OTA server or recovery to ensure
the correct OTA package is used for update. We haven't made any
restrictions on what ro.boot* variables can be used for fingerprint
override. One possible candidate can be the skus listed in
ODM_MANIFEST_SKUS.

Bug: 152167826
Test: unittests pass, generate an OTA file with the new option
Change-Id: I637dea3472354236d2fd1ef0a3306712b3283c29