Allow factory specific kernel commandline

* BOARD_KERNEL_CMDLINE_FACTORY_BOOT can be set to
add kernel commandline items to the factory ramdisk,
such as fiq_debugger.console_enable=Y

Change-Id: Iee4060b96eaeecc517975c56342fd5445b1feabf
Signed-off-by: Nick Sanders <nsanders@google.com>