FROMLIST: bootconfig: Share the checksum function with tools

Move the checksum calculation function into the header for sharing it
with tools/bootconfig.

Picking this up FROMLIST to get the fix ASAP. It is likely to be
accepted upstream.

Signed-off-by: Devin Moore <devinmoore@google.com>
Link: https://lore.kernel.org/lkml/162262192121.264090.6540508908529705156.stgit@devnote2/


Bug: 183237066
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Change-Id: Ifd79a0fc7d75bde733ffe8f979c7e30e4c5844c7