cpu/stm32: unify cpu_init()

cpu/stm32_common/cpu_init.c

cpu/stm32_common/stmclk_l0l1.c

cpu/stm32f0/cpu.c

cpu/stm32f2/cpu.c

cpu/stm32f3/cpu.c

cpu/stm32f4/cpu.c

cpu/stm32f7/cpu.c

cpu/stm32l1/cpu.c

cpu/stm32l4/cpu.c