LP-41 - Add support for ccache

- explicitly define QMAKE_CC & CXX as "ccache <compiler>"
- use ccache for uavobjectgenerator
- Handle CCACHE setting with configuration options

To enable ccache you need to configure it like
  make CCACHE=/path/to/ccache config_append

ground/openpilotgcs/openpilotgcs.pri

@@ -248,3 +248,9 @@ macx {
         QMAKE_CXXFLAGS_WARN_ON += -Wno-gnu-static-float-init
     }
 }
+
+
+# use ccache when available
+QMAKE_CC = $$(CCACHE) $$QMAKE_CC
+QMAKE_CXX = $$(CCACHE) $$QMAKE_CXX
+

ground/uavobjgenerator/uavobjgenerator.pro

@@ -8,6 +8,10 @@ QT -= gui
 macx {
     QMAKE_CXXFLAGS  += -fpermissive
 }
+# use ccache when available
+QMAKE_CC = $$(CCACHE) $$QMAKE_CC
+QMAKE_CXX = $$(CCACHE) $$QMAKE_CXX
+
 TARGET = uavobjgenerator
 CONFIG += console
 CONFIG -= app_bundle

make/firmware-defs.mk

@@ -21,8 +21,8 @@ ifndef TOP_LEVEL_MAKEFILE
 endif
 
 # Define toolchain component names.
-CC      = $(ARM_SDK_PREFIX)gcc
+CC      = $(CCACHE) $(ARM_SDK_PREFIX)gcc
-CXX     = $(ARM_SDK_PREFIX)g++
+CXX     = $(CCACHE) $(ARM_SDK_PREFIX)g++
 AR      = $(ARM_SDK_PREFIX)ar
 OBJCOPY = $(ARM_SDK_PREFIX)objcopy
 OBJDUMP = $(ARM_SDK_PREFIX)objdump

make/tools.mk

@@ -204,6 +204,8 @@ endif
 # Command to extract version info data from the repository and source tree
 export VERSION_INFO = $(PYTHON) $(ROOT_DIR)/make/scripts/version-info.py --path=$(ROOT_DIR)
 
+export CCACHE
+
 ##############################
 #
 # Misc settings