edbg.inc.mk: handle IMAGE_OFFSET being defined with a space

Handle that `IMAGE_OFFSET` can be set to something that contain spaces.

With 'IMAGE_OFFSET=$$((0x1000 + 0x1000))' we had in the command line when doing
'flash'

    --offset $((0x1000 --offset + --offset 0x1000))

With the change we correctly have

    --offset $((0x1000 + 0x1000))

makefiles/tools/edbg.inc.mk

@@ -12,7 +12,7 @@ ifneq (,$(DEBUG_ADAPTER_ID))
 endif
 
 # Set offset according to IMAGE_OFFSET if it's defined
-EDBG_ARGS += $(addprefix --offset ,$(IMAGE_OFFSET))
+EDBG_ARGS += $(if $(IMAGE_OFFSET),--offset $(IMAGE_OFFSET))
 
 FFLAGS ?= $(EDBG_ARGS) -t $(EDBG_DEVICE_TYPE) -b -v -p -f $(HEXFILE)