Fix PATH setup for arm64.

On architectures other than "arm", the combination of
ANDROID_KERNEL_TOOLCHAIN_PATH being empty, and CODE_REVIEWS being empty,
leads to adding a :: into the user's PATH, which is highly undesirable.

Neither CODE_REVIEWS nor the mips toolchaindir is ever used, so just
remove them, and make sure we only include the extra colon when setting
the kernel toolchain path to a non-empty value.

Change-Id: I2dfa7d3a322b56b6abbc47476082dc6ae4dd6a82