QEMU: Enable package for MIPS targets

Allow QEMU to be installed on MIPS targets. Also introduce a new
symbol (BR2_PACKAGE_QEMU_ARCH_SUPPORTS_TARGET) to hold the
architectures than can build and use QEMU on the target.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>