mk: add 'upload' target to copy fresh ELF to Linux board using SCP

`make pxf upload` builds ELF and copy it to target
device using scp.

Target hostname should be specified in BOARD_LINUX_HOST variable

mk/board_linux.mk

@@ -5,3 +5,4 @@ include $(MK_DIR)/find_tools.mk
 # Linux build is just the same as SITL for now
 LIBS = -lm -lpthread -lrt
 include $(MK_DIR)/board_native.mk
+include $(MK_DIR)/upload_firmware.mk

mk/upload_firmware.mk

@@ -0,0 +1,11 @@
+ifneq ($(BOARD_LINUX_HOST),)
+upload: $(SKETCHELF).timestamp-upload
+
+$(SKETCHELF).timestamp-upload: $(SKETCHELF)
+	scp $(SKETCHELF) $(BOARD_LINUX_HOST):
+	touch $@
+else
+upload:
+	@echo Check your config.mk: BOARD_LINUX_HOST should be defined to upload firmware
+	exit 1
+endif