Add ccache support

Author: LDong-Arm

.travis.yml

@@ -18,6 +18,16 @@ language: sh
 os: linux
 dist: focal
 
+cache:
+  pip: true
+  ccache: true
+  directories:
+    # Cache arm-none-eabi compiler
+    - ${HOME}/.cache/deps
+    # It looks like ccache for arm-none-eabi is not yet supported by Travis.
+    # Therefore manually adding ccache directory to cache
+    - ${HOME}/.ccache
+
 matrix:
   include:
 
@@ -37,6 +47,14 @@ matrix:
         - export PATH="$PATH:${PWD}/gcc-arm-none-eabi-9-2020-q2-update/bin"
         - popd
         - arm-none-eabi-gcc --version
+        # Setup ccache
+        - ccache -o compiler_check=content
+        - ccache -M 1G
+        - pushd /usr/lib/ccache
+        - sudo ln -s $(which ccache) arm-none-eabi-gcc
+        - sudo ln -s $(which ccache) arm-none-eabi-g++
+        - export PATH="/usr/lib/ccache:$PATH"
+        - popd
         # Fetch mbed-os: We use manual clone, with depth=1 and --single-branch to save time.
         - git clone --depth=1 --single-branch https://github.com/ARMmbed/mbed-os.git;
         # Install Mbed CLI and dependencies