[PATCH] Cleanup SLHC configuration

Convert selection of serial line header compression to use CONFIG_SLHC
rather than makefile ifeq uglyness.  This makes it easier to select
the SLHC module from other code.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>