package/libcoap: bump to version 4.3.0

- disable examples source which is enabled by default since
  https://github.com/obgm/libcoap/commit/feb8c15b4572569716cf1c037092edb4717ca537
- mbedtls is an optional dependency since
  https://github.com/obgm/libcoap/commit/f601ddbf78720a74f29acb704256bc2655c4aeaa
- Add Apache-2.0 for mbedtls to LICENSE:
  https://github.com/obgm/libcoap/commit/0fccf2e71347af0ef3973ac565fca110ec3d4f40

Since mbedtls is not bundled but comes from the Buildroot package, we
don't need to declare the Apache-2.0 license for libcoap. It only
applies to mbedtls.

https://github.com/obgm/libcoap/blob/v4.3.0/ChangeLog



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>