[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