build common .o files everywhere (fixes issue #29)

vector.o, nvic.o, scb.o and assert.o are available on every platform,
but at least some of them differ between the implementations. they
already got built explicityly on some platforms; now adding them to the
common Makefile.include.

lib/lpc13xx/Makefile

@@ -28,7 +28,7 @@ CFLAGS		= -Os -g -Wall -Wextra -I../../include -fno-common \
 		  -ffunction-sections -fdata-sections -MD
 # ARFLAGS	= rcsv
 ARFLAGS		= rcs
-OBJS		= gpio.o assert.o vector.o
+OBJS		= gpio.o
 
 VPATH += ../cm3