buildroot

lua: ensure pkgconfig directory exists before installing files

The lua staging and host installation commands generate a file in
usr/lib/pkgconfig, without first making sure that this directory
exists, which causes build failures if it doesn't. This commit adjusts
those installation commands to create this directory if needed.

Fixes:

  http://autobuild.buildroot.net/results/101c89e1d6aee942a0b1c4e4f3daf8ac2414a56c/

Based on investigation and initial (more complicated) fix provided by
Francois Perrad <francois.perrad@gadz.org>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>