fwgen.py: Use binascii.crc32()

Use the crc32 implementation from the binascii package instead of our own.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Rename fwgen.py to sigmadsp_fwgen

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
sigmadsp_fwgen: Add simple help text

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>