ThinkNode M6/M1 room_server: set ROOM_PASSWORD default

Without ROOM_PASSWORD the guest password defaults to empty, and a client
sending a blank password is granted read+write access (can post). Every
other board's room_server env sets ROOM_PASSWORD; the ThinkNode M6 and M1
were the only two shipping an open room. Default them to "hello" to match
the rest of the tree; operators can still change it at runtime with
'set guest.password'.

variants/thinknode_m1/platformio.ini

@@ -58,6 +58,7 @@ build_flags =
   -D ADVERT_LAT=0.0
   -D ADVERT_LON=0.0
   -D ADMIN_PASSWORD='"password"'
+  -D ROOM_PASSWORD='"hello"'
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1
 build_src_filter = ${ThinkNode_M1.build_src_filter}

variants/thinknode_m6/platformio.ini

@@ -63,6 +63,7 @@ build_flags =
   -D ADVERT_LAT=0.0
   -D ADVERT_LON=0.0
   -D ADMIN_PASSWORD='"password"'
+  -D ROOM_PASSWORD='"hello"'
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1
 build_src_filter = ${ThinkNode_M6.build_src_filter}