Build: Add Custom Toolchain Support

Since arm-eabi-4.8 breaks many legacy devices (ie: they won't boot or
won't build) I've implemented custom toolchain support. We at least
needed a 4.7 option which this commit allows. Also, many of the newer
devices do better on newer toolchains (4.9 for example) this commit just
makes sense.

How to enable: Add "TARGET_GCC_VERSION_ARM := 4.7" (change 4.7 to 4.9 or
even 4.9-sm etc) to the BoardConfig.mk of your device. Make sure that
you have toolchains located in prebuilts/gcc/linux-x86/arm/ and that
they have arm-eabi- on the front of them.

Example: arm-eabi-4.9-sm is your folder name so TARGET_GCC_VERSION_ARM
:= 4.9-sm is your BoardConfig.mk entry.