HBSD: Optionally include src.opts.mk.

By using .sinclude, the build framework will utilize src.opts.mk for
base, but not for ports, since src.*.mk does not get installed into
/usr/share/mk. This is needed to change the PIE knob from make.conf(5)
to src.conf(5).

Obtained-from:	@bdrewery <bdrewery@freebsd.org>
Signed-off-by:	Shawn Webb <shawn.webb@hardenedbsd.org>

share/mk/bsd.init.mk

@@ -13,6 +13,7 @@ __<bsd.init.mk>__:
 .if exists(${.CURDIR}/../Makefile.inc)
 .include "${.CURDIR}/../Makefile.inc"
 .endif
+.sinclude <src.opts.mk>
 .include <bsd.own.mk>
 .MAIN: all
 beforebuild: .PHONY .NOTMAIN