emulation: Restructure the Kconfig
At present ARM and x86 use differently named symbols to mean similar things. For x86, board/emulation/Kconfig contains the targets, but ARM has them in a different file. While ARM uses ARCH_QEMU to mean QEMU, x86 uses VENDOR_EMULATION.
This series adjusts the Kconfig for emulation targets on x86 and ARM, to improve consistency.
A new arch-neutral MACH_QEMU is introduced. This is enabled for all ARM/x86 emulation targets. Furture work could add this for other archs too.