libethdrivers,zynq: Fix more issues in driver

This commit includes a number of fixes to the Ethernet driver:
- Set the TX descriptor once and only once. The descriptor only needs to
be set once and the DMA controller is able to keep track of where it is
in the ring.
- Set the 'wrap' bit in the beginning for the descriptors.
- Check 'used' bit in the first descriptor in complete_(tx/rx). Only
the first descriptor has the bit set when the Ethernet device is done
with the descriptor chain.
- Fix typo in descriptor size assignment.

trivial: Fixed style

imx6/zynq7000: Add functions for getting MAC addr

1. Add get_mac function to raw_iface_funcs so Ethdriver
components can get device MAC address

2. Remove the code for configuring custom MAC address

Libethdrivers: Remove unnecessary header files

The libraries that the headers belong to are not linked to libethdrivers.
There is also no usage of the libraries inside.

Added Set/Clear Promiscuous mode functionality

Change-Id: I3cd5fc49ed8a5215fc6f1068a8a0fa0288a1ab1f

Modified zynq Ethernet source code to function with seL4/CAmkES

Change-Id: I817c1af1b51d58a017532a0f75e86905a5947000

Grabbed zynq specific files from Xilinx's u-boot fork, commit b904725.

Modified some common i.MX6 files with Xilinx u-boot specific changes.

https://github.com/Xilinx/u-boot-xlnx/tree/b904725a112f7b7a1b5d657541d77dc8fa1a1615

Change-Id: Ib90372873f7e2dc8d2a268735326ef9d97468bc0