makefile: handle -n / -k / -q correctly

Use $(findstring) instead of $(filter) to detect -n/-k
as different versions of MAKE fill in $(MAKEFLAGS) differently.
Do not bother running ninja at all if -nq is passed.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20201026155854.3074290-1-pbonzini@redhat.com>