programmer: Add Developerbox/CP2104 bit bang driver

The 96Boards Developerbox (a.k.a. Synquacer E-series) provides a CP2102
debug UART with its GPIO pins hooked up to the SPI NOR FLASH. The
circuit is intended to provide emergency recovery functions without
requiring any additional tools (such as a JTAG or SPI programmer). This
was expected to be very slow (and it is) but CP2102 is much cheaper than
a full dual channel USB comms chip.

Read performance is roughly on par with a 2400 baud modem (between 60
and 70 minutes per megabyte if you prefer) and write performance is 50%
slower still. The full recovery process, with backup and verification of
4MB data written takes between 14 and 15 hours. Thus it is only really
practical as an emergency recovery tool, firmware developers will need
to use an alternative programmer.

Change-Id: I2547a96c1a2259ad0d52cd4b6ef42261b37cccf3
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
---
M Makefile
A developerbox_spi.c
M flashrom.c
M programmer.h
4 files changed, 278 insertions(+), 2 deletions(-)