framework: add new variable PKGCONFIG_BASE

The base system started to provide certain pkgconfig files for
components it provides. This new variable allows ports to prefer base
components explicitly using for example

PKGCONFIG_BASE=foo

This will create a symlink of /usr/libdata/pkgconfig/foo.pc to
${WKRDIR}/.pkgconfig and adding said path to pkgconfigs search path.

This allows us to switch easily between base and port implementation
of different components (say openssl) by simply defining the appropriate
variable in (say ssl.mk).

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35494