How to compile modules with debug symbols

 Hi, everyone. I just learned how to develop modules 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.

There are variables in `sys/conf/kmod.mk` which control the compiling
process. But I did not find anyone that can help to add debug symbols.

Thanks.