AndroidKernel.mk: Fix multiple recompile with Android mm macro

Skip header file generation section if KERNEL_DEFCONFIG
parameter is not defined.  This occurs during Android mm
build of a module (without dependency); and the lack
of KERNEL_DEFCONFIG causes the command line to trigger a
kernel recompile.

Change-Id: I709d8c23691442e537ec6e2b7aebeb010d90a139
Signed-off-by: David Ng <dave@codeaurora.org>