build: sort Makefile assignment list entries

Sort Makefile assignment list entries so that it's easier to track
what's added, removed, or missing.

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
include: move include/* into include/ofono/

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
build: accommodate to headers move

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>
plugins: replace libdl with gmodule

Signed-off-by: You-Sheng Yang <vicamo@gmail.com>