cmake: qemu: Don't use the '\$' hack with Ninja

Using the '\$' technique to accept make variables corrupts Ninja build
systems, so avoid using it when generating Ninja files. Not using it
with Ninja means we need to come up with some other mechanism to
support build-time configuration of the QEMU CLI with Ninja.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>