1. In original driver, the stack size for emac driver task "emacT" is only 2048 Bytes, which also can not be changed by user.
2. Each time when invoking "emac_start" function, it will reset emac registers, the driver should reconfig some important registers again.
1. Hello World application shows no footprint difference before and
after this change
2. examples/ethernet/ethernet application compiles properly (can't
test with my board)
1. The transmitting mode of the packets from LWIP to MAC is changed from synchronous to asynchronous.
2. The receive buf mode : support pointer mode and copy mode.
3. Add get phy status func used to config mac register.
