security/wpa_supplicant*: Fix EAPOL_TEST build for non-standard SSL_DEFAULT

The post-build target does not have the same environment variables setup
as the do-build target. This results in the eapol_test binary getting
compiled against the headers of the TLS library specified in
SSL_DEFAULT, but getting linked against the TLS library from the base
system regardless of SSL_DEFAULT, causing build failures when
SSL_DEFAULT is set to a different TLS library/version than the on from
the base system.

Fix this by not using the post-build target to build the eapol_test
binary, but instead conditionally add the eapol_test target to
TARGET_ALL if the EAPOL_TEST option is enabled for the port.

Signed-off-by: Pascal Ernster <git@hardfalcon.net>