STM32 EMAC Driver Rewrite Complete!

[multiplemonomials]

Just merged a huge update to the STM32 Ethernet drivers. This PR basically rewrites them completely from scratch, using a new, much more ergonomic ethernet MAC driver API. See the design document for more details on how the new driver works.

This should solve a number of issues that have been annoying people for years with the STM32H7 Ethernet driver, including:

• Very slow transmissions (due to blocking until packets have been completely transmitted)
• Slowing down the entire MCU by about a factor of 2 when Ethernet is first enabled (due to disabling the data cache)
• No multicast join support
• Ethernet dying if you tried to transmit a packet buffer located in DTCM, or another memory location that the Ethernet DMA can't access
• No support for Nanostack
• No support for running the Ethernet test suite (so I couldn't even tell what other more subtle issues it might have had)
• No support for ethernet PHYs other than LAN8742

And while the STM32F2/F4/F7 Ethernet driver wasn't nearly as bad, it still was not zero copy and had a few of the above issues, so I rewrote it as well. Now, both drivers are fully zero-copy and should be implemented in the most performant way possible! (P.S. STM32H5 now supports Ethernet as well).

If you use Ethernet on an STM32 MCU, I would highly recommend updating to the latest commit of mbed-os and giving it a shot. If you use STM32F7 or H7, you should see an immediate performance benefit. Please let me know if you see any regressions or other issues!

P.S. There are breaking changes if and only if you use a custom target that provides its own Ethernet pinmap init code. See the PR for details.