lib/crc8: Add crc start value

To make the usage of this function more flexible, lets add the CRC start
value as parameter to this function. This way it can be used by other
functions requiring different start values than 0 as well.

For non-zero CRC start values to work, I've reworked the function a bit.
The new implementation is copied from the Linux version in
drivers/i2c/i2c-core.c / i2c_smbus_pec(). Which supports non-zero
CRC stating values.

I've double-checked that the results for zero starting values are
identical to the results from the original version of this function.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>