build: reorganize usb a bit

src/transport/build.mk

@@ -1,7 +1,4 @@
 SRCS += \
-	transport/transport.c \
-	transport/usb/descriptor.c \
-	transport/usb/report.c \
-	transport/usb/usb.o
+	transport/transport.c
 
-obj/transport/usb/descriptor.o obj/transport/usb/report.o obj/transport/usb/usb.o: CPPFLAGS += -I ext/tinyusb/src -D CFG_TUSB_CONFIG_FILE=\"$(ROOT)/src/transport/usb/tusb_config.h\"
+include $(ROOT)/src/transport/usb/build.mk

src/transport/usb/build.mk

@@ -0,0 +1,6 @@
+SRCS += \
+	transport/usb/descriptor.c \
+	transport/usb/report.c \
+	transport/usb/usb.c
+
+CPPFLAGS += -I ext/tinyusb/src -D CFG_TUSB_CONFIG_FILE=\"../../tusb_config.h\"