make: buildtest: don't rm BINDIRBASE, make clean all

Makefile.buildtests

@@ -38,7 +38,6 @@ buildtest:
 	@ \
 	BUILDTESTOK=true; \
 	APP_RETRY=0; \
-	rm -rf "$$BINDIRBASE"; \
 	for BOARD in $$($(MAKE) -s info-boards-supported); do \
 	  RIOTNOLINK=$$(echo $(BOARD_INSUFFICIENT_MEMORY) | grep $${BOARD} 2>&1 >/dev/null && echo 1); \
 	  ${COLOR_ECHO} -n "Building for $${BOARD} "; \
@@ -63,7 +62,7 @@ buildtest:
 	        WERROR=$${WERROR} \
 	        LTO=$${LTO} \
 	        TOOLCHAIN=$${TOOLCHAIN} \
-	        $(MAKE) -j$(NPROC) -B 2>&1) ; \
+	        $(MAKE) -j$(NPROC) clean all 2>&1) ; \
 	    if [ "$${?}" = "0" ]; then \
 	      ${COLOR_ECHO} "${COLOR_GREEN}success${COLOR_RESET}"; \
 	      if [ -n "$${BUILDTEST_VERBOSE}" ]; then \