[build] use -ggdb3 DEBUG_FLAGS by default

-ggdb3 make slightly bigger .elf files, but allows gdb to understand
macros, which paparazzi uses somewhat extensively.
Make this the default, since it only impacts the size of the debug sections and
not the size of the flashed binary (in the lpc21/stm32 case).
On the linux system we really don't care about these few more bytes...

conf/Makefile.nps

@@ -27,6 +27,8 @@
 SRC_ARCH = arch/sim
 
 OPT ?= 2
+# Slightly bigger .elf files but gains the ability to decode macros
+DEBUG_FLAGS ?= -ggdb3
 
 # Launch with "make Q=''" to get full command display
 Q=@
@@ -36,7 +38,7 @@ CFLAGS += $(INCLUDES)
 CFLAGS += $($(TARGET).CFLAGS)
 CFLAGS += $(USER_CFLAGS)
 CFLAGS += -O$(OPT)
-CFLAGS += -g
+CFLAGS += $(DEBUG_FLAGS)
 CFLAGS += -std=gnu99
 CFLAGS += $(shell pkg-config --cflags-only-I ivy-glib)
 
@@ -45,7 +47,7 @@ CXXFLAGS += $(INCLUDES)
 CXXFLAGS += $($(TARGET).CFLAGS)
 CXXFLAGS += $(USER_CFLAGS)
 CXXFLAGS += -O$(OPT)
-CXXFLAGS += -g
+CXXFLAGS += $(DEBUG_FLAGS)
 CXXFLAGS += $(shell pkg-config --cflags-only-I ivy-glib)
 
 LDFLAGS	+= $($(TARGET).LDFLAGS)