boards: enabled C++ for arduino-due and udoo

boards/arduino-due/Makefile.features

+FEATURES_PROVIDED += cpp
 FEATURES_PROVIDED += periph_uart periph_gpio periph_spi periph_random

boards/arduino-due/Makefile.include

@@ -4,6 +4,7 @@ export CPU = sam3x8e
 # define tools used for building the project
 export PREFIX = arm-none-eabi-
 export CC = $(PREFIX)gcc
+export CXX = $(PREFIX)g++
 export AR = $(PREFIX)ar
 export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc

boards/udoo/Makefile.features

+FEATURES_PROVIDED += cpp
 FEATURES_PROVIDED += periph_uart periph_gpio periph_spi periph_random

boards/udoo/Makefile.include

@@ -4,6 +4,7 @@ export CPU = sam3x8e
 # define tools used for building the project
 export PREFIX = arm-none-eabi-
 export CC = $(PREFIX)gcc
+export CXX = $(PREFIX)g++
 export AR = $(PREFIX)ar
 export AS = $(PREFIX)as
 export LINK = $(PREFIX)gcc
@@ -38,6 +39,10 @@ export OFLAGS += -O binary
 export FFLAGS += -R -e -w -v -b bin/$(BOARD)/$(APPLICATION).hex
 export TERMFLAGS += -p "$(PORT)"
 
+# unwanted (CXXUWFLAGS) and extra (CXXEXFLAGS) flags for c++
+export CXXUWFLAGS +=
+export CXXEXFLAGS +=
+
 # use the nano-specs of the NewLib when available
 ifeq ($(shell $(LINK) -specs=nano.specs -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
 export LINKFLAGS += -specs=nano.specs -lc -lnosys