tests: limit concurrency level

Some Travis CI machines have 32 CPUs. This sets our concurrency level to 33.
Travis CI kills our buildtest for obvious reasons.

This PR limits the concurrency level to 8 on Travis CI.

.travis.yml

 language: c
 
+env:
+    - NPROC_MAX=8
+
 before_install:
     - sudo apt-get install emdebian-archive-keyring
     - echo 'deb http://www.emdebian.org/debian wheezy main' | sudo tee /etc/apt/sources.list.d/emdebian.list > /dev/null
@@ -27,6 +30,7 @@ install:
     - git log -1 --pretty=format:%H riot/master
 
 script:
+    - make -s -C ./examples/default info-concurrency
     - git rebase riot/master || git rebase --abort
 
     - ./dist/tools/compile_test/compile_test.py

Makefile.buildtests

-ifneq (, $(filter buildtest, $(MAKECMDGOALS)))
+ifneq (, $(filter buildtest info-concurrency, $(MAKECMDGOALS)))
   ifeq (, $(strip $(NPROC)))
     # Linux (utility program)
     NPROC := $(shell nproc 2>/dev/null)
@@ -17,6 +17,13 @@ ifneq (, $(filter buildtest, $(MAKECMDGOALS)))
     endif
 
     NPROC := $(shell echo $$(($(NPROC) + 1)))
+
+    ifneq (, $(NPROC_MAX))
+      NPROC := $(shell if [ ${NPROC} -gt $(NPROC_MAX) ]; then echo $(NPROC_MAX); else echo ${NPROC}; fi)
+    endif
+    ifneq (, $(NPROC_MIN))
+      NPROC := $(shell if [ ${NPROC} -lt $(NPROC_MIN) ]; then echo $(NPROC_MIN); else echo ${NPROC}; fi)
+    endif
   endif
 endif
 
@@ -190,3 +197,6 @@ info-build:
 
 info-boards-supported:
 	@echo "$(BOARDS)"
+
+info-concurrency:
+	@echo "$(NPROC)"