README: cleanup requirements for pkg-config
FreeBSD fully supports pkg-config; the .pc files for the internal
libusb has been added with
https://cgit.freebsd.org/src/commit/?id=
041d3f3f09b8
and became part of FreeBSD 10.0 in 2014-01-16.
Remove the obsoleted requirements for adding .pc files.
While there, add pkgconf as an alternative to pkg-config.
Change-Id: I16aea735c44107cb71945f225a979682c8c92d0a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7255
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins