backports: modify kconfig parser to use bpid and target_dir

backports: modify kconfig parser to use bpid and target_dir

The kconfig parser sets a base directory using the directory
in which the Kconfig file passed lives in. This strategy assumes the
kernel uses relative paths, while this is truly supported in
practice though upstream only deals with full paths based on
the actual root directory of the kernel. Its safer, cleaner and
much simpler to support full paths, and since the kernel uses
full paths anyway lets stick to that and complain when relative
paths are found. This also modifies our own provided Kconfig to
use full paths as well, we'll need this later to adjust our
Kconfigs for kernel integration.

$ 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    44m3.002s
user    1188m20.708s
sys     140m25.540s

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