wpa: Restructure wpa build

The current WPA build assumes a flat namespace. However the latest sources
from w1.fi now have a duplicate config.c, in two separate subdirectories.
The flat namespace will overwrite config.o with the output from the most
recently modified config.c, of which there are two of them.

This commit resolves this problem by building each component in
wpa's src subdirectory tree into its own .a archive, just as the w1.fi
upstream build as used by the port does. The advantages of this approach
are:

1. Duplicate source file names, i.e. config.c in the wpa_supplicant
direcory and another config.c in src/utils in the next wpa
will result in both compiles writing to the same .o file.

2. This restructure simplifies maintanence. A develper needs only to add
new files as identified by git status in the vendor branch to the
appropriate Makefile within the usr.sbin/wpa tree. This also reduces
time required to prepare a new import and should reduce error.

3. The new wpa build structure more closely represents the build as
performed by the upstream tarball.

This is in preparation for the next wpa update from w1.fi.

Reviewed by: philip
Tested by: philip
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D30372