Fix qemu image definition conditionals

BOARD_USES_<X>IMAGE is not equivalent with that image actually
existing. Since these targets depend on the INSTALLED_<X>IMAGE
variables being defined, gate the conditionals on that instead.

This logic is used in other places in Makefile

Bug: 120974093
Test: make
Change-Id: I6766042c801bf7fddc4b84671dbe5d3a157aea6b