cpu: cortexm*: move cpu_conf.h stuff from cpu.h into cpu_conf_common.h

cpu/cc2538/include/cpu_conf.h

cpu/cortexm_common/include/cpu.h

cpu/cortexm_common/include/cpu_conf_common.h

cpu/ezr32wg/include/cpu_conf.h

cpu/k60/include/cpu_conf.h

cpu/k64f/include/cpu_conf.h

cpu/kw2x/include/cpu_conf.h

cpu/lm4f120/include/cpu_conf.h

cpu/lpc11u34/include/cpu_conf.h

cpu/lpc1768/include/cpu_conf.h

cpu/nrf51/include/cpu_conf.h

cpu/nrf52/include/cpu_conf.h

cpu/sam3/include/cpu_conf.h

cpu/samd21/include/cpu_conf.h

cpu/saml21/include/cpu_conf.h

cpu/stm32f0/include/cpu_conf.h

cpu/stm32f1/include/cpu_conf.h

cpu/stm32f3/include/cpu_conf.h

cpu/stm32f4/include/cpu_conf.h

cpu/stm32l1/include/cpu_conf.h