backports: make kconfig src_line backport / ConfigTree specific

backports: make kconfig src_line backport / ConfigTree specific

The kconfig source line used on a backport project is very
project specific. For backport packaging the source line simply
should follow the full path of the project but for integration
this will vary depending on the backport prefix directory used.
We'll add support for that next.

$ 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    43m56.439s
user    1187m4.516s
sys     138m53.328s

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