[gentoo-hardened] Update on SELinux eclass to support different git repo or branch

[Sven Vermeulen]

Hi all

Our live sec-policy/selinux-* packages (the ones with the -9999 version)
have been using our git repository for some time. Although users could
always override these with packagename_LIVE_REPO, it meant that they had to
generate such variables for each and every sec-policy/ package out there.

As that is a big nuisance (and you should also not forget to substitute "-"
with "_" in that case) I added two more variables in the SELinux eclass to
make this a lot simpler:

SELINUX_GIT_REPO can now point to a different repo set if you want
SELINUX_GIT_BRANCH can be used to pull in a different branch (default is
master)

The first one (SELINUX_GIT_REPO) allows users to use their own SELinux
policy repository with the live ebuilds, rather than having to use ours (or
do some cludgy things to get their own policies to be loaded). All that we
require is that the repository is still a reference policy clone.

The second one (SELINUX_GIT_BRANCH) allows users to pick a different branch.
This can be interesting when we're doing larger updates on the SELinux
policies with wider testing (or in between developers).

The changes are fully backwards compatible and do not affect users not using
the -9999 ebuilds.

Wkr,
	Sven Vermeulen