selftest/classes: add localpkgfeed class

This class can be used to construct a subset of a deployed package feed
for use in tests which iterate the deploy directory, and as such a huge
feed of 30K+ packages can result in very slow tests.

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