quilt: rewrite ptest glue

The ptest glue was overly complex. Not as many files are needed to be
installed if we tell it to use the system's quilt tools, there's no
need for a custom Makefile just to set some environment variables, and
/usr/bin/ed doesn't appear to be used at all.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>