travis: cache compilers

This currently removes around 25s from compilation time while also being
more resilient to network outages. The drawback is that when changing
the compiler we may need to clean the caches.

.travis.yml

@@ -1,6 +1,10 @@
 language: cpp
 sudo: false
 
+cache:
+  directories:
+    - $HOME/opt
+
 addons:
   apt:
     sources:
@@ -25,19 +29,30 @@ addons:
     branch_pattern: coverity_scan
 
 before_install:
-  - pushd .
-    && cd ~
-    && wget http://firmware.diydrones.com/Tools/PX4-tools/gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
-    && tar -xf gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
-    && wget http://firmware.diydrones.com/Tools/Travis/NavIO/master.tar.gz
-    && tar -xf master.tar.gz
-    && mkdir -p $HOME/bin
+  - pushd $HOME
+  - pushd $HOME/opt
+  # PX4 toolchain
+  - compiler="gcc-arm-none-eabi-4_9-2015q3"
+  - if [ ! -d "$HOME/opt/$compiler" ]; then
+      wget http://firmware.diydrones.com/Tools/PX4-tools/gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
+      && tar -xf gcc-arm-none-eabi-4_9-2015q3-20150921-linux.tar.bz2
+      ;
+    fi
+  # RPi toolchain
+  - compiler="tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64"
+  - if [ ! -d "$HOME/opt/$compiler" ]; then
+      wget http://firmware.diydrones.com/Tools/Travis/NavIO/master.tar.gz
+      && tar -xf master.tar.gz
+      ;
+    fi
+  - popd
+  - mkdir -p $HOME/bin
     && ln -sf /usr/bin/gcc-4.8 $HOME/bin/gcc
     && ln -sf /usr/bin/g++-4.8 $HOME/bin/g++
-    && exportline="export PATH=$HOME/bin:$HOME/gcc-arm-none-eabi-4_9-2015q3/bin:$HOME/tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin:\$PATH"
+    && exportline="export PATH=$HOME/bin:$HOME/opt/gcc-arm-none-eabi-4_9-2015q3/bin:$HOME/opt/tools-master/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/bin:\$PATH"
     && if grep -Fxq "$exportline" ~/.profile; then echo nothing to do ; else echo $exportline >> ~/.profile; fi
     && . ~/.profile
-    && popd
+  - popd
 
 script:
   - Tools/scripts/build_all_travis.sh