buildroot

p7zip: new package

Only enabled on a subset of known-working architectures.
It fails to build on some archs, like blackfin.

Signed-off-by: André Hentschel <nerv@dawncrow.de>
[Thomas:
 - Add comment about the Blackfin exception.
 - Propagate the Blackfin exception to the Config.in comment.
 - Fix the license, it is the LGPLv2.1+ *with* the unRAR restriction.
 - Use $(INSTALL) -D -m 0755 to install the binary instead of cp.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>