fix: use rsvg-convert as the conversion backend

The auxiliary pdf build requires SVG files to be converted to pdf files.
Use libRSVG as a light weight alternative to inkscape, and ensure it's
installed in the build environment.

Change-Id: I1fdb05fb2701fc28a04e210d5928f4387ca08613
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>