Merge pull request #6907 from haukepetersen/add_board_nucleo144-f746

boards/nucleo144-f746/Makefile

boards/nucleo144-f746/Makefile.dep

boards/nucleo144-f746/Makefile.features

boards/nucleo144-f746/Makefile.include

boards/nucleo144-f746/board.c

boards/nucleo144-f746/dist/openocd.cfg

boards/nucleo144-f746/include/board.h

boards/nucleo144-f746/include/periph_conf.h

core/include/panic.h

cpu/cortexm_common/cortexm_init.c

cpu/cortexm_common/include/vectors_cortexm.h

cpu/cortexm_common/thread_arch.c

cpu/cortexm_common/vectors_cortexm.c

cpu/stm32_common/cpu_common.c

cpu/stm32_common/include/periph_cpu_common.h

cpu/stm32_common/include/stmclk.h

cpu/stm32_common/periph/uart.c

cpu/stm32f7/Makefile

cpu/stm32f7/Makefile.include

cpu/stm32f7/cpu.c