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-linux-androideabi-4.6
  arm-linux-androideabi-4.7

 also available subject to merge of http://gerrit.aokp.co/13756 : -
  arm-eabi-4.8
  arm-linux-androideabi-4.8


Change-Id: Ifbac6e93b98929807e89b980c9600cb733b4798d
Signed-off-by: Arnav Gupta <championswimmer@aokp.co>
build: no longer build emulator by default

This is just increasing build times when most people are not using it.
Let's speed up builds a bit.

Building emulator should still be possible by appending BUILD_EMULATOR=true
to the build command

PS2: Let's keep the darwin warning.

Change-Id: Ibd8e2b90455bd603a691c7b95466068b6fae212a
Signed-off-by: Andrew Warfield <hiasant@gmail.com>
Merge "build: kernel: allow usage of custom toolchain for kernel build" into kitkat
Merge branch 'kitkat' of https://github.com/AOKP/build into kitkat