Distinguish boot images by target rather than arch

We plan to add boot image variants for host tests.
Distinguishing the variants by arch does not work,
since both host and device can have the same arch.

(cherry picked from commit c177ebec)

Test: m
Bug: 152037801
Merged-In: Iea73c77367affb074f97a0fc318389417ce537da
Change-Id: I8b95882bb00cd2fd1f6cfd8a2784f9ebb957eec6