sb: Add pkg-config support.

Add a pkg-config look alike command so packages that use pkg-config can
build if pkg-config is not present on a host.

Add support to query package config from configuration scripts.
config: Update qemu to build on FreeBSD.

Update configuration to build on FreeBSD. This is based on pkg-config
detecting the packages installed on the host.
sb: Remove the extra path. Use the default.