Name of exit function in exit.c

Change the name of the exit function to _exit as suggested in clib
documenation for all gcc_arm toolchains (GCC_ARM and CoIDE)
[NUCLEO_F0] Add PeripheralPins files
[NUCLEO_F103RB] Add PeripheralPins files
[NUCLEO_F3] Add PeripheralPins files
[NUCLEO_F4] Add note on optional pins
[NUCLEO_L0/L1] Add PeripheralPins files
[NUCLEO_L053R8] Correct issue with Travis build
Merge pull request #738 from bcostm/master

Targets: NUCLEOs - Add PeripheralPins files for all nucleo targets
Merge pull request #711 from ohagendorf/_exit-or-exit

mbed: Name of exit function in exit.c for GCC ARM (_exit)
[ARCH_MAX & DISCO_F407VG] add SystemCoreClockUpdate and HAL_Init to
mbed_sdk_init

HAL_Init is needed by Arch MAX to setup RTC with internal clock
Merge pull request #739 from xiongyihui/master

Targets: ARCH_MAX & DISCO_F407VG - Add SystemCoreClockUpdate and HAL_Init to mbed_sdk_init