pkgbase: Fix building out-of-tree manual pages

c7e6cb9e08d6 introduced MK_MANSPLITPKG but it was not available for
building out-of-tree manual pages. For example, x11/nvidia-driver fails
with the following error:

> doc (all)

make[3]: "/usr/share/mk/bsd.man.mk" line 53: Malformed conditional (${MK_MANSPLITPKG} == "no")
make[3]: Fatal errors encountered -- cannot continue

Move the definition from src.opts.mk to bsd.opts.mk to make it visible.

(cherry picked from commit 6827435548d257c672f934db5c6ff01012d96995)