make: don't overwrite LINKFLAGS

Fix the boards' Makefiles were a potentially set LINKFLAGS variable were
overwritten.

boards/chronos/Makefile.include

@@ -14,7 +14,7 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-export LINKFLAGS = -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
+export LINKFLAGS += -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
 export FLASHER = mspdebug
 export USEMODULE += msp430_common
 export FFLAGS = rf2500 "prog $(HEXFILE)"

boards/mbed_lpc1768/Makefile.include

@@ -11,8 +11,8 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-#LINKFLAGS =  -g3 -ggdb -mcpu=cortex-m3 $(FPU_USAGE) -mlittle-endian -static -lgcc -mthumb -mthumb-interwork -nostartfiles -T$(RIOTCPU)/$(CPU)/LPC1768.ld
-LINKFLAGS = -mcpu=cortex-m3 -mthumb -Wl,--gc-sections,--cref -lc -lgcc -lnosys -T$(RIOTCPU)/$(CPU)/LPC1768.ld -nostartfiles
+#LINKFLAGS +=  -g3 -ggdb -mcpu=cortex-m3 $(FPU_USAGE) -mlittle-endian -static -lgcc -mthumb -mthumb-interwork -nostartfiles -T$(RIOTCPU)/$(CPU)/LPC1768.ld
+LINKFLAGS += -mcpu=cortex-m3 -mthumb -Wl,--gc-sections,--cref -lc -lgcc -lnosys -T$(RIOTCPU)/$(CPU)/LPC1768.ld -nostartfiles
 
 ifeq ($(strip $(PORT)),)
 	export PORT = /dev/ttyUSB0

boards/msb-430-common/Makefile.include

@@ -12,7 +12,7 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-export LINKFLAGS = -mmcu=$(MCU) -lgcc
+export LINKFLAGS += -mmcu=$(MCU) -lgcc
 TERMPROG = $(RIOTBASE)/dist/tools/pyterm/pyterm
 
 export FLASHER ?= mspdebug

boards/msba2-common/Makefile.include

@@ -14,7 +14,7 @@ export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
 FLASHER = lpc2k_pgm
 TERMPROG ?= $(RIOTBASE)/dist/tools/pyterm/pyterm
-LINKFLAGS = -gdwarf-2 -mcpu=arm7tdmi-s -static -lgcc -nostartfiles -T$(RIOTBASE)/cpu/$(CPU)/linkerscript.x
+LINKFLAGS += -gdwarf-2 -mcpu=arm7tdmi-s -static -lgcc -nostartfiles -T$(RIOTBASE)/cpu/$(CPU)/linkerscript.x
 
 ifeq ($(strip $(PORT)),)
 	export PORT = /dev/ttyUSB0

boards/redbee-econotag/Makefile.include

@@ -19,7 +19,7 @@ export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
 FLASHER = mc1322x-load.pl
 TERM = pyterm
-LINKFLAGS = -mcpu=arm7tdmi-s -static -lgcc -nostartfiles -T$(RIOTBASE)/cpu/$(CPU)/mc1322x.lds
+LINKFLAGS += -mcpu=arm7tdmi-s -static -lgcc -nostartfiles -T$(RIOTBASE)/cpu/$(CPU)/mc1322x.lds
 
 ifeq ($(strip $(PORT)),)
 	export PORT = /dev/ttyUSB0

boards/telosb/Makefile.include

@@ -13,7 +13,7 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-export LINKFLAGS = -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
+export LINKFLAGS += -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
 export TERMPROG = $(RIOTBASE)/dist/tools/pyterm/pyterm
 export FLASHER = $(RIOTBASE)/dist/tools/goodfet/goodfet.bsl
 ifeq ($(strip $(PORT)),)

boards/wsn430-common/Makefile.include

@@ -12,7 +12,7 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-export LINKFLAGS = -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
+export LINKFLAGS += -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
 export FLASHER = mspdebug
 ifeq ($(strip $(PORT)),)
     export PORT = /dev/ttyUSB0

boards/z1/Makefile.include

@@ -13,7 +13,7 @@ export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
 export SIZE = $(PREFIX)size
 export OBJCOPY = $(PREFIX)objcopy
-export LINKFLAGS = -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
+export LINKFLAGS += -mmcu=$(MCU) -lgcc $(BINDIR)msp430_common/startup.o
 export TERMPROG = $(RIOTBASE)/dist/tools/pyterm/pyterm
 export FLASHER = $(RIOTBASE)/dist/tools/goodfet/goodfet.bsl
 ifeq ($(strip $(PORT)),)