make: use shared DEBUG_FLAGS (-gdb3) by default

-ggdb3 make slightly bigger .elf files, but allows gdb to understand
macros, which libopenocm3 uses somewhat extensively.  Make this the
default, and pull it up to the common base makefile, so it can be easily
substituted.

lib/sam/3u/Makefile

@@ -25,9 +25,10 @@ PREFIX		?= arm-none-eabi
 
 CC		= $(PREFIX)-gcc
 AR		= $(PREFIX)-ar
-CFLAGS		= -Os -g -Wall -Wextra -I../../../include -fno-common \
+CFLAGS		= -Os -Wall -Wextra -I../../../include -fno-common \
 		  -mcpu=cortex-m3 -mthumb $(FP_FLAGS) -Wstrict-prototypes \
 		  -ffunction-sections -fdata-sections -MD -DSAM3U
+CFLAGS          += $(DEBUG_FLAGS)
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs
 OBJS		= gpio_common_all.o gpio_common_3a3u3x.o pmc.o usart.o