How to compile modules with debug symbols

 Hi, everyone. I just learned how to develop device drivers in FreeBSD.
A simple one with this Makefile.
```
KMOD=   hello
SRCS=   hello.c

.include <bsd.kmod.mk>
```
Type `make` and it works. But no debug symbols by default.

I followed `bsd.kmod.mk` and found `sys/conf/kmod.mk`. There are dozens of
variables in `sys/conf/kmod.mk` which control the compiling process. But I
did not find one that helps to add debug symbols or add `-g` into the
CFLAGS.

Thanks.