Fixes for runqemu script an OVMF recipe

This series fixes:  
* Blank screen when using runqemu with the OVMF firmware (albiet in a somewhat hacky way)  
* OVMF recipe not warning the user if compiled for unsupported architectures like MIPS  
* OVMF recipe never creating 32-bit firmware, even when it should  

I chose not to include the workaround for GCC 4.9 in this series, because  
* luv-yocto doesn't have GCC 4.9 anyway, so that change is better applied when upstreaming the OVMF recipe to oe-core. 
* OVMF builds correctly with GCC 4.9 _only when the target architecture is 32 bits._ Enabling GCC 4.9 exposes users to build failures when building for 64-bit targets.