[Buildroot] [git commit master] buildroot; move defconfigs to configs/ and print in help

[Peter Korsgaard]

commit: http://git.buildroot.net/buildroot/commit/?id=58fd779a4355e5bfa2bc0b15ed9c4664f44c7a0f
branch: http://git.buildroot.net/buildroot/commit/?id=refs/heads/master

As discussed earlier on the mailing list. It simplifies code, gives more
sensible error message on typos and makes the defconfigs easier to find
for users.

Furthermore, update documentation to match.

Signed-off-by: Peter Korsgaard <jacmet at sunsite.dk>

Patch is too large, so refusing to show it