This adds support for JTAG programming by bitbanging GPIOs exposed on
the RaspberryPi's expansion header.

Tested by connecting directly to an STM32VLDiscovery board, without any
additional circuity. I observed maximum about 4MHz on the TCK pin with an
old analogue 'scope and about 100kHz when setting the speed to 100kHz.
Busyloop waiting is needed because even with a single 0ns nanosleep call
(with FIFO priority) it lowers the TCK speed to ~30kHz which is way too low
to be useful.

The speed testing with adapter_khz 2000 gave the following results:
sudo chrt -f 1 nice -n -19 ./src/openocd \
     -f interface/raspberrypi-native.cfg \
     -c "set WORKAREASIZE 0x2000" \
     -f target/stm32f1x.cfg -c "adapter_khz 2000"

wrote 131072 bytes from file random.bin in 3.973677s (32.212 KiB/s)
dumped 131072 bytes in 1.445699s (88.538 KiB/s)

This is 3.7 times faster for writing and 14.3 times faster for reading
compared to the generic sysfsgpio driver; probably the writing speed is
limited by the target itself here and reading speed might be considerably
higher too with appropriate connection and a capable target.

BCM2835 name is choosen over BCM2708 because the published peripherals
datasheet uses the particular model name and not family name.

Change-Id: Ib78168be27f53c2a3c88c3dd8154d1190c318c78
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/758
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>