mchp: convert pack_ec.py script to Python 3
mchp: convert pack_ec.py script to Python 3

Some changes to bytes/string handling to convert to Python 3. Most of
these changes came from the mec1322 script (since it seems large
sections were copy/pasted).

BUG=chromium:1031705
BRANCH=none
TEST=make BOARD=reef_mchp, ensure binary equivalent

Change-Id: Idffa2dbef7359aa176c1aafa1c504e9e29ee9d49
Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1997736
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec