kernel build: Get rid of the if else chain

Manifest updated so all toolchains are in the paths like
prebuilts/gcc/$(TARGET_ARCH)/arm/$(TARGET_KERNEL_CUSTOM_TOOLCHAIN)/bin/arm-eabi-

Boardconfig.mk should only need
TARGET_KERNEL_CUSTOM_TOOLCHAIN := arm-eabi-4.X
or for linaro
TARGET_KERNEL_CUSTOM_TOOLCHAIN := Linaro-4.7

Change-Id: Idf22fff4592f4cede1a29c434695c4e79beeee6d