Add gatekeeper app to userspace library build system

Switch to building gatekeeper with the trusted_app.mk system instead of
as a generic lk module. This allows the app to depend on common
userspace static libraries.

Test: build.py qemu-generic-arm64-test-debug
Bug: 169448829
Change-Id: I1798bd8ae04d6c748a6e5fa038965496e43a7745
1 file changed