backports: allow for different backport prefix and simple integration support

backports: allow for different backport prefix and simple integration support

The way we backport when packaging is to minimize the amount of
changes required by taking advantage of the fact that Kconfig
can treat CONFIG_ as an environment variable with getenv()
when parsing menu entries. When doing integration we don't want
to do this so instead we'll rely on the CONFIG_BACKPORT prefix.
This requires a bit of work on our current parsers, so to do that
identify the backport and provide that to helpers.

This also provides initial basic support for integration where no
additional code was required other than prefixes or ensuring certain
things are not done for integration. Before we expose integration
support we'll need to add quite a bit more things so this just assumes
integration is never desired.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>