wpa_supplicant: rework .config file editing

Do all substitutions on the .config file in a single sed command.
Make the .config file editing a lot more readable. Based on ideas
brought up by Arnout Vandecappelle on the mailing-list.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
 (build-test with all features enabled)
[Samuel: rebase on head of master]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>