tools/edbg: allow Makefile to be called stand-alone

Also:
- refactor the Makefile so all is not called if edbg is already there
- add a clean target to only remove the edbg binary

dist/tools/edbg/Makefile

@@ -3,14 +3,23 @@ PKG_URL=https://github.com/ataradov/edbg
 PKG_VERSION=99d15460fcff723f73b16c29c8ca14bff4b33b20
 PKG_LICENSE=BSD-3-Clause
 
+# manually set some RIOT env vars, so this Makefile can be called stand-alone
+RIOTBASE ?= $(CURDIR)/../../..
+RIOTTOOLS ?= $(CURDIR)/..
+
 PKG_SOURCE_DIR = $(CURDIR)/bin
 PKG_BUILD_OUT_OF_SOURCE = 0
 
 include $(RIOTBASE)/pkg/pkg.mk
 
-all:
+all: $(CURDIR)/edbg
+
+$(CURDIR)/edbg:
 # Start edbg build in a clean environment, so variables set by RIOT's build process
 # for cross compiling a specific target platform are reset and edbg can
 # be built cleanly for the native platform.
 	env -i PATH="$(PATH)" TERM="$(TERM)" "$(MAKE)" -C "$(PKG_BUILD_DIR)"
 	mv $(PKG_BUILD_DIR)/edbg .
+
+clean::
+	rm -f $(CURDIR)/edbg