iotile-emulate OSX "-L" enhancement

OSX needs a workaround for the qemu-system-gnuarmeclipse emulator so it can find device SVDs.

Note from @timburke
> good to add this fix into `iotile-emulate` so that on mac it automatically tries to find the right folder relative to where `qemu` binary is installed, then `iotile-build` can use `iotile-emulate` rather than directly invoking `qemu` and benefit from the automatic workaround