Added option for config file.

Things the can be added to config file are:
export PATH := /usr/lib/ccache/bin:$(PATH)
GCS_OPTS := XXX
etc

.gitignore

@@ -3,6 +3,10 @@
 /tools
 /build
 /3rdparty
+
+# Ignore user config
+config
+
 
 # Exclude temporary and system files
 Thumbs.db

Makefile

@@ -140,6 +140,9 @@ else ifeq ($(UNAME), Windows)
     UAVOBJGENERATOR = $(BUILD_DIR)/uavobjgenerator/uavobjgenerator.exe
 endif
 
+CONFIG_FILE := config
+-include $(CONFIG_FILE)
+
 ##############################
 #
 # All targets