build: kernel: allow usage of custom toolchain for kernel build

 set using
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := arm-eabi-4.6

 available options are
  arm-eabi-4.6
  arm-eabi-4.7
  arm-eabi-4.8
  arm-linux-androideabi-4.6
  arm-linux-androideabi-4.7
  arm-linux-androideabi-4.8

Change-Id: Ifbac6e93b98929807e89b980c9600cb733b4798d
Signed-off-by: Arnav Gupta <championswimmer@aokp.co>