AndroidKernel: add support to pass build flags

Add support to pass build flags while compiling Linux kernel.

This change is required to add support in Linux kernel to use external
host executable. (example: dtc)

Usage example:
TARGET_KERNEL_MAKE_ENV := DTC_EXT=dtc$(HOST_EXECUTABLE_SUFFIX)

Change-Id: I008ca00cf9695133ff167ffdee8d8dfb89beb598
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>