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

'flash' also works by flashing both the bootloader and slot0
independently.

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

makefiles/boot/riotboot.mk

@@ -84,7 +84,7 @@ riotboot/flash-bootloader: riotboot/bootloader/flash
 # avoid circular dependency against clean
 riotboot/bootloader/%: $$(if $$(filter riotboot/bootloader/clean,$$@),,$$(BUILDDEPS) pkg-prepare)
 	$(Q)/usr/bin/env -i \
-		QUIET=$(QUIET) PATH="$(PATH)"\
+		QUIET=$(QUIET) PATH="$(PATH)" USER="$(USER)"\
 		EXTERNAL_BOARD_DIRS="$(EXTERNAL_BOARD_DIRS)" BOARD=$(BOARD)\
 		DEBUG_ADAPTER_ID=$(DEBUG_ADAPTER_ID) \
 		IOTLAB_NODE=$(IOTLAB_NODE) \
@@ -155,7 +155,22 @@ riotboot/flash: riotboot/flash-slot0 riotboot/flash-bootloader
 # make applications that use the riotboot feature default to actually using it
 # Target 'all' will generate the combined file directly.
 # It also makes 'flash' and 'flash-only' work without specific command.
-FLASHFILE = $(RIOTBOOT_EXTENDED_BIN)
+#
+# Special case for uf2conv where bootloader and slot0 are both flashed
+# independently
+ifneq (,$(filter uf2conv,$(PROGRAMMER)))
+  ifneq (,$(filter riotboot/flash-extended-slot0 riotboot/flash-combined-slot0,$(MAKECMDGOALS)))
+    $(error riotboot/flash-extended-slot0 riotboot/flash-combined-slot0 are not supported with uf2conv)
+  endif
+  FLASHFILE = $(BOOTLOADER_BIN)/riotboot.bin
+  riotboot/flash-slot0-remount: riotboot/flash-slot0
+	sleep $(TERM_DELAY)
+	$(PREFLASHER_PREFIX)$(PREFLASHER) $(PREFFLAGS)
+	sleep $(PREFLASH_DELAY)
+  flash: riotboot/flash-slot0-remount
+else
+  FLASHFILE = $(RIOTBOOT_EXTENDED_BIN)
+endif
 
 else
 riotboot:

makefiles/tools/uf2conv.inc.mk

@@ -1,7 +1,9 @@
 FLASHFILE ?= $(HEXFILE)
 
+IMAGE_OFFSET ?= $(ROM_OFFSET)
+
 FLASHER ?= $(RIOTTOOLS)/uf2/uf2conv.py
-FFLAGS  ?= $(UF2CONV_FLAGS) $(FLASHFILE)
+FFLAGS  ?= $(UF2CONV_FLAGS) $(FLASHFILE) --base $(IMAGE_OFFSET)
 
 PREFLASH_DELAY ?= 2