makefiles/tools/uf2conv.inc.mk: partial riotboot support

Individual files need to be converted to uf2 format, targets
flashing individual slots or the bootloader will work:

- riotboot/flash-slot%
- riotboot/flash-bootloader

But not targest flashing combined/extended versions since conversion
of the blob is not possible with the uf2conv.py script.

makefiles/boot/riotboot.mk

@@ -78,7 +78,7 @@ riotboot: $(SLOT_RIOT_BINS)
 riotboot/flash-bootloader: riotboot/bootloader/flash
 riotboot/bootloader/%: $(BUILDDEPS)
 	$(Q)/usr/bin/env -i \
-		QUIET=$(QUIET) PATH="$(PATH)"\
+		QUIET=$(QUIET) PATH="$(PATH)" USER="$(USER)"\
 		EXTERNAL_BOARD_DIRS="$(EXTERNAL_BOARD_DIRS)" BOARD=$(BOARD)\
 		DEBUG_ADAPTER_ID=$(DEBUG_ADAPTER_ID) \
 		PROGRAMMER=$(PROGRAMMER) PROGRAMMER_QUIET=$(PROGRAMMER_QUIET) \

makefiles/tools/uf2conv.inc.mk

@@ -6,8 +6,10 @@ ifneq (,$(FFLAGS_OPTS))
   UF2CONV_FLAGS ?= $(FFLAGS_OPTS)
 endif
 
+IMAGE_OFFSET ?= $(ROM_OFFSET)
+
 FLASHER ?= $(RIOTTOOLS)/uf2/uf2conv.py
-FFLAGS  ?= $(UF2CONV_FLAGS) $(FLASHFILE)
+FFLAGS  ?= $(UF2CONV_FLAGS) $(FLASHFILE) --base $(IMAGE_OFFSET)
 
 PREFLASH_DELAY ?= 2