make: supply `-j` on `buildtest`

We support `make -j` for a while now. `make buildtest` takes ages to
finish. So just supply `-j` on buildtest.

Makefile.buildtests

@@ -1,3 +1,25 @@
+ifneq (, $(filter buildtest, $(MAKECMDGOALS)))
+  ifeq (, $(strip $(NPROC)))
+    # Linux (utility program)
+    NPROC := $(shell nproc 2>/dev/null)
+
+    ifeq (, $(strip $(NPROC)))
+      # Linux (generic)
+      NPROC := $(shell grep -c ^processor /proc/cpuinfo 2>/dev/null)
+    endif
+    ifeq (, $(strip $(NPROC)))
+      # BSD (at least FreeBSD and Mac OSX)
+      NPROC := $(shell sysctl -n hw.ncpu 2>/dev/null)
+    endif
+    ifeq (, $(strip $(NPROC)))
+      # Fallback
+      NPROC := 1
+    endif
+
+    NPROC := $(shell echo $$(($(NPROC) + 1)))
+  endif
+endif
+
 buildtest:
 	@if [ -z "$${JENKINS_URL}" ] && tput colors 2>&1 > /dev/null; then \
 		GREEN='\033[1;32m'; RED='\033[1;31m'; RESET='\033[0m'; \
@@ -29,7 +51,7 @@ buildtest:
 			RIOTBOARD=$${RIOTBOARD} \
 			RIOTCPU=$${RIOTCPU} \
 			BINDIRBASE=$${BINDIRBASE} \
-			$(MAKE) 2>&1 >/dev/null) ; \
+			$(MAKE) -j$(NPROC) 2>&1 >/dev/null) ; \
 		if [ "$${?}" = "0" ]; then \
 			$${ECHO} "$${GREEN}success$${RESET}"; \
 		else \