em100: Add SPI trace console

Add a command which allows a realtime console output from em100
when using the CONFIG_SPI_FLASH_CONSOLE option in coreboot.

Provide the offset and lenght of the region defined in the FMAP
and it will watch for the host to write to that area, and when
it sees a write it will print it to stdout.

Unlike the already implemented SPI terminal function, this does
not rely on the host sending EM100 specific commands (that might
be filtered out by the SPI controller or the ME)

Change-Id: I78162585bcd73a3bb3a8aa590f43a96092946109
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
---
M em100.c
M em100.h
M trace.c
3 files changed, 161 insertions(+), 8 deletions(-)