MAC address

[sstaub]

The build in mac address should used. Following function can used, based on mbed-os/stm32xx_emac.c

`
void stmMAC(uint8_t *mac) {
// Read unic id
#if defined (STM32F2)
uint32_t baseUID = *(uint32_t *)0x1FFF7A10;
#elif defined (STM32F4)
uint32_t baseUID = *(uint32_t *)0x1FFF7A10;
#elif defined (STM32F7)
uint32_t baseUID = *(uint32_t *)0x1FF0F420;
#else
#error MAC address can not be derived from target unique Id
#endif

mac[0] = 0x00;
mac[1] = 0x80;
mac[2] = 0xE1;
mac[3] = (baseUID & 0x00FF0000) >> 16;
mac[4] = (baseUID & 0x0000FF00) >> 8;
mac[5] = (baseUID & 0x000000FF);
return;
}
`

[fpistm]

Hi @sstaub ,
good idea.
Do not hesitate to submit a PR for that or any other contributor ;)

[sstaub]

I did make a PR #11