netdev_rx_err() and netdev_tx_complete_err() get passed the error
code, but currently use it only in debug messages.

Retain error numbers and frequencey counts for up to
NETDEV_MAX_UNIQUE_ERRORS (4) different errors for each of TX and RX.
This allows the "ifstat" command to report the reasons for TX/RX
errors in most cases, even in non-debug builds.

This implementation is tested and working, but not currently tied in
to the EFI build.

We have EFI APIs for CPU I/O, PCI I/O, timers, console I/O, user
access and user memory allocation.

EFI executables are created using the vanilla GNU toolchain, with the
EXE header handcrafted in assembly and relocations generated by a
custom efilink utility.