scripts/runqemu: Fix logic error causing failures with MACHINE from the environment

scripts/runqemu: Fix logic error causing failures with MACHINE from the environment

MACHINE=qemux86-64 oe-selftest -r runqemu.RunqemuTests.test_boot_machine_slirp_qcow2

fails yet

oe-selftest -r runqemu.RunqemuTests.test_boot_machine_slirp_qcow2

with MACHINE in local.conf would work.

It turns out that:

runqemu slirp wic.qcow2 qemux86-64

works but:

MACHINE=qemux86-64 runqemu slirp wic.qcow2 qemux86-64

does not.

The reason are the misplaced return statements in runqemu, its skipping a block
of logic when MACHINE is set in the environment when it shouldn't. Fix this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>