kernel: allow to specify a custom toolchain for kernel build

TARGET_KERNEL_CUSTOM_TOOLCHAIN := arm-eabi-4.4.3

Also removed workaround for darwin (I6321fb7f6814b207c821d974766d945351b3f546)
since the binaries got merged

Change-Id: I12f3587ce7c85ee6d3741f0619a376b13a936a90
build: add support for uncompressed kernels