build: use a config.mk if it exists, but don't require it

mk/environ.mk

@@ -53,6 +53,11 @@ ifneq ($(findstring CYGWIN, $(SYSTYPE)),)
     SKETCHBOOK	:= $(shell cygpath ${SKETCHBOOK})
 endif
 
+ifneq ($(wildcard $(SKETCHBOOK)/config.mk),)
+$(warning Reading $(SKETCHBOOK)/config.mk)
+include $(SKETCHBOOK)/config.mk
+endif
+
 #
 # Work out the sketch name from the name of the source directory.
 #

mk/px4_targets.mk

@@ -1,6 +1,8 @@
 # PX4 build is via external build system
 
+ifeq ($(PX4_ROOT),)
 PX4_ROOT=../PX4Firmware
+endif
 
 # cope with relative paths
 ifeq ($(wildcard $(PX4_ROOT)/nuttx-configs),)