Build: Add Custom Kernel Toolchain Support

* Many legacy devices don't support GCC 4.8 on the kernel (ie they won't boot or build) so we at least need a 4.7 option which this commit can easily provide. Also, many of the newer devices do better either Linaro 4.8 or SM 4.8-4.10 so 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.

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>