Merge pull request #2432 from recrof/kiss-radio-everywhere

kiss radio: added stm32 support + envs in platformio.ini

variants/rak3x72/platformio.ini

@@ -41,3 +41,8 @@ build_src_filter = ${rak3x72.build_src_filter}
   +<../examples/companion_radio/*.cpp>
 lib_deps = ${rak3x72.lib_deps}
   densaugeo/base64 @ ~1.4.0
+
+[env:RAK_3x72_kiss_modem]
+extends = rak3x72
+build_src_filter = ${rak3x72.build_src_filter}
+  +<../examples/kiss_modem/>