common/{hush, kgdb, serial}.c: build by COBJS-$(...) in Makefile

Move global '#ifdef CONFIG_xxx .... #endif' out of the .c files and into
the COBJS-$(CONFIG_xxx) in the Makefile.  Also delete unused var in kgdb
code in the process.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>