net: lib: lwm2m: fix coap proxy resource option

commit 2a7546fb ("net: lwm2m: add support for coap2coap proxy")
erroneously changed the COAP_OPTION_* used to specify the coap2coap
or coap2http proxy resource used from COAP_OPTION_URI_PATH to
COAP_OPTION_PROXY_SCHEME.

Changing it back to COAP_OPTION_URI_PATH requires us to re-order how
the coap options are appended to the packet as the coap options must
be added in the order specified by the numbers in:
https://tools.ietf.org/html/rfc7252#section-12.2



Signed-off-by: Michael Scott <michael@opensourcefoundries.com>