libcoap: Fix gnutls/openssl/mbedtls PACKAGECONFIG conflicts

These were missing a comma so were being added as RRECOMMENDS.

Original patch is from Alex Kiernan <alex.kiernan@gmail.com>.
Upstream commit in meta-openembedded:
    3589aa5debbf59e940588939dd913130e4cfd4cd 
    ("libcoap: Fix gnutls/openssl/mbedtls PACKAGECONFIG conflicts")

Importing the patch because the recipe is not present in 
kirkstone branch but only in master branch of meta-openembedded.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Francesco Pham <francesco.pham@huawei.com>