backports: use BACKPORT_DIR prefix on kconfig sources

backports: use BACKPORT_DIR prefix on kconfig sources

This will allow us to do less work for built-in integration support.
When packaging we only use BACKPORT_DIR for Kconfig sources we control,
to avoid a large change we avoid doing this for other Kconfig files.

For integration we'll always have to use BACKPORT_DIR on kconfig sources.

Suporting requires a bit of self evaluation of the variable within our
kconfig library, ideally we'd have support for groking all variables
defined within the Kconfig setup but that requires quite a lot more
work as it means also parsing Makefiles and inheriting these definitions
within config symbols when used. Since we only define one right now and
its used for built-in support we deal with it ourselves for now.

Please consider the complexity of adding others, it doesn't seem like
there would be much need for others though. If others wanted to use a
different BACKPORT_DIR path other than 'backports' that would require
tweaking here, but we'll start by assuming no one will want to do that.

$ time /home/mcgrof/backports/devel/ckmake --allyesconfig
1   3.0.101             [  OK  ]
2   3.1.10              [  OK  ]
3   3.2.62              [  OK  ]
4   3.3.8               [  OK  ]
5   3.4.104             [  OK  ]
6   3.5.7               [  OK  ]
7   3.6.11              [  OK  ]
8   3.7.10              [  OK  ]
9   3.8.13              [  OK  ]
10  3.9.11              [  OK  ]
11  3.10.58             [  OK  ]
12  3.11.10             [  OK  ]
13  3.12.31             [  OK  ]
14  3.13.11             [  OK  ]
15  3.14.22             [  OK  ]
16  3.15.10             [  OK  ]
17  3.16.6              [  OK  ]
18  3.17.1              [  OK  ]
19  3.18-rc1            [  OK  ]

real    42m41.825s
user    1187m38.880s
sys     141m11.760s

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