mako: Use GCC 4.7 for kernel builds

GCC 4.8 seems to be consistently creating broken kernels for APQ8064
targets, and also causing numerous problems for OMAP4s with Samsung
eMMC

Also add GCC 4.7 prebuilt toolchains to omni.dependencies

May be related to http://permalink.gmane.org/gmane.linux.ports.arm.omap/119412

Change-Id: I94a0de9953410ce5cd4c7be6a92fa93981de6481
overlay: Disable currently unsupported options

Change-Id: I71f4e192235ac33d733098722b12ba482f663220