Build: rework custom toolchain selection

kernel.mk: whitespace sanitation + dumpvar + USE_AOSP_TOOLCHAIN handling

custom kernel's can now be chosen by setting TARGET_KERNEL_CUSTOM_TOOLCHAIN := linaro-4.x
the cpu for optimization is automaticly set by TARGET_CPU_VARIANT

Change-Id: I17e49219337dbac8f9fb4b51895adbb1ee83a4bd

Conflicts:
	core/dumpvar.mk
	core/tasks/kernel.mk