more options for kernel toolchain selection:
if 'TOOLCHAIN_GNUEABIHF:= true' in device tree, arm-linux-gnueabihf- is used instead of arm-eabi- (which is the default)

also don't change default kernel toolchain (can be changend by setting TARGET_KERNEL_CUSTOM_TOOLCHAIN:=LINARO-kernel, to use Cl3Kener's linaro kernel toolchain

Change-Id: If7bcb5925da4904d8b082a698d9ef4fa3399660b