ROMFS: add support for EXTERNAL_SCRIPTS

this adds support for an EXTERNAL_SCRIPTS directory, to complement the
EXTERNAL_APPS option. It allows external apps to install scripts in
ROMFS

ROMFS/Makefile

@@ -35,6 +35,10 @@ ROMFS_FSSPEC	:= $(SRCROOT)/scripts/rcS~init.d/rcS \
 		   $(SRCROOT)/mixers/FMU_octo_+.mix~mixers/FMU_octo_+.mix \
 		   $(SRCROOT)/logging/logconv.m~logging/logconv.m
 
+# the EXTERNAL_SCRIPTS variable is used to add out of tree scripts
+# to ROMFS. 
+ROMFS_FSSPEC += $(EXTERNAL_SCRIPTS)
+
 #
 # Add the PX4IO firmware to the spec if someone has dropped it into the
 # source directory, or otherwise specified its location.