Extended #476, which does not break ethernet for K64F

Diff from #476:
- Merged with the latest mbed master - few conflicts resolved manually (MCU_K64F folder for example)
- Enet driver + osa + os abstraction layer + phy driver - reverted to previous KSDK version (commit cfd2a98 for future reference)

The reverted enet will be reverted back once lwip arch sources for K64F gets update to the KSDK 1.0 enet driver. There are couple of warnings present, which will be resolved by an update of enet drivers.

I'll close Erik's request #476 , as this is an extension with resolving conflicts. Thanks Erik for helping out!