Add recovery_available to prebuilt_etc.

The property is similar to the ones for cc_*, where it can additionally
create a recovery variant of the module when used.

Bug: 112780007
Test: Build mke2fs.conf with `recovery_available: true`.
Test: `go test -run TestPrebuiltEtc`
Change-Id: Ia04360d00453fa1f80022301feb369514f5e2b0b