wpa_supplicant: mesh support needs openssl

When BR2_PACKAGE_WPA_SUPPLICANT_MESH_NETWORKING is enabeld,
wpa_supplicant currently fails to build with:

../src/common/sae.o: In function `sae_cn_confirm.isra.2':
sae.c:(.text+0x168): undefined reference to `crypto_bignum_to_bin'
sae.c:(.text+0x198): undefined reference to `crypto_bignum_to_bin'
../src/common/sae.o: In function `sae_cn_confirm_ffc':
sae.c:(.text+0x214): undefined reference to `crypto_bignum_to_bin'
sae.c:(.text+0x22c): undefined reference to `crypto_bignum_to_bin'
../src/common/sae.o: In function `sae_cn_confirm_ecc':
sae.c:(.text+0x2a4): undefined reference to `crypto_ec_point_to_bin'
sae.c:(.text+0x2c0): undefined reference to `crypto_ec_point_to_bin'

[...]

This is due to the fact that the SAE code, used for the mesh network
support, needs OpenSSL support. Therefore, we ensure that
BR2_PACKAGE_WPA_SUPPLICANT_MESH_NETWORKING selects OpenSSL. Only
OpenSSL is supported, which is why
BR2_PACKAGE_OPENSSL_FORCE_LIBOPENSSL is selected as well.

No changes to the .mk files are needed, because we were already
handling OpenSSL as an optional dependency.

This problem was not yet caught by the autobuilders.

Reported-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>