kernel build: add a new flag $(TARGET_KERNEL_CUSTOM_TOOLCHAIN_SUFFIX

Used to define the name of the binaries used to compile. Usually this is arm-eabi but with
custom toolchains this sometimes needs to be changed. Allow that to be done.

Must be used in conjunction with TARGET_KERNEL_CUSTOM_TOOLCHAIN

Change-Id: I4d6850c96573b5959dc421e0049c9fb7e88db105