wpa-supplicant: Simplify build/install flow

wpa-supplicant: Simplify build/install flow

The build/install steps for wpa-supplicant support CFLAGS and an install
target, so use these rather than attempting to do it manually (which was
broken by upstream splitting into build.rules and lib.rules).

Note that this installs wpa_passphrase into sbindir rather than bindir.

Signed-off-by: Alex Kiernan <alexk@zuma.ai>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>