FBSD: allow exclusion of kernel symbols

FBSD: allow exclusion of kernel symbols

Gerrit/5503 added support for kernel module symbols in the
bsd.kmod.mk world; however, the inclusion of symbols is conditoinal
on the DEBUG_FLAGS make variable being *set*, as opposed to nonempty.
So we must use an intermediate variable to store the AC_SUBST
result and conditionalize assignment to DEBUG_FLAGS on whether
or not it is empty.

Change-Id: I7f86fd08d6dcf1ac7116680db98db13f5f31be7b
Reviewed-on: http://gerrit.openafs.org/5507
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>