zephyr: zmake: Separate out the argument parser from main function
zephyr: zmake: Separate out the argument parser from main function

Move the code which generates the argument parser to a separate
function, get_argparser.  This is intended that we can access the
argument parser to generate documentation about zmake's options and
subcommands.

BUG=b:180609783
BRANCH=none
TEST=unit tests pass

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I1f14aa4e298d0ec47fb51e7b3bfad52bab990e85
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3382478
Reviewed-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>

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