Fix LOCAL_MODULES and improve the make output.

Fix LOCAL_MODULES and improve the make output.

The exists() check guarding the invocation of ls was not working
correctly as it was expanding '$L' to determine the path of the local
modules directory.  Fix by using {} around the variable name.

Inline some of the logic from bsd.subdir.mk when invoking local module
builds.  This gives output in 'make buildkernel' the same as if there
was a Makefile in /usr/local/sys/modules with SUBDIR =
${LOCAL_MODULES}.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D20991