pkg/nordic_softdevice_ble: reset memory in the .hex file

softdevice needs the memory at 0x2000 to be initialized to 0xffffffff
according to #5893 and testing. However, the addresses [0x8bc, 0x3000[ are not
set in softdevice.hex.

So use a modified hex file with all the memory set to 0xff as it is the rom
reset value anyway.

This change updates the `.hex` file instead on relying on erasing the
memory.

boards/common/nrf52/Makefile.include

@@ -21,7 +21,7 @@ ifeq (jlink,$(PROGRAMMER))
 
   # special options when using SoftDevice
   ifneq (,$(filter nordic_softdevice_ble,$(USEPKG)))
-    export JLINK_PRE_FLASH := erase\nloadfile $(BINDIR)/softdevice.hex
+    export JLINK_PRE_FLASH := loadfile $(BINDIR)/softdevice.hex
     export FLASH_ADDR := 0x1f000
     export LINKER_SCRIPT ?= $(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL)_sd.ld
     # murdock: softdevice.hex file is used for flashing

pkg/nordic_softdevice_ble/Makefile

@@ -21,8 +21,11 @@ prepare: $(PKG_SRCDIR)/.extracted
 $(BINDIR)/ble_6lowpan.a: $(PKG_SRCDIR)/.extracted
 	cp $(PKG_SRCDIR)/$(BLE_6LOWPAN_LIB) $@
 
+# softdevice.hex has the `[0x8bc, 0x3000[` addresses not set.
+# However, it requires that the value at `0x2000` is 0xFFFFFFFF
+# We just put all the undefined memory to 0xff as it is the rom reset value anyway.
 $(BINDIR)/softdevice.hex: $(PKG_SRCDIR)/.extracted
-	cp $(PKG_SRCDIR)/$(SOFTDEVICE) $@
+	$(Q)$(OBJCOPY) $(OFLAGS) -Oihex --gap-fill 0xff $(PKG_SRCDIR)/$(SOFTDEVICE) $@
 
 $(PKG_SRCDIR)/.extracted: $(PKG_BUILDDIR)/$(PKG_FILE)
 	rm -rf $(@D)