coap: fix override syntax

- convert to kirkstone override syntax
- restrict example package to include only examples directory

Signed-off-by: Francesco Pham <francesco.pham@huawei.com>

meta-oniro-staging/recipes-connectivity/coap/coap_git.bb

@@ -36,4 +36,4 @@ EXTRA_OECONF = "--disable-doxygen --disable-manpages --with-mbedtls"
 BBCLASSEXTEND = "native nativesdk"
 
 PACKAGE_BEFORE_PN = "${PN}-examples"
-FILES_${PN}-examples = "${datadir}/*"
+FILES:${PN}-examples = "${datadir}/libcoap/examples/*"