Update init.rc prebuilt handling

This CL modifies the build system to skip checking of init.rc files if
TARGET_BUILD_UNBUNDLED is set.  This is necessary because several of the
artifacts required by the host_init_verifier program aren't present in
unbundled builds.

Test: m
Change-Id: I37be43af81057c5adddc1b80a04d74a090183bc7