zmake: add documentation about where the args for Zmake class comes from
zmake: add documentation about where the args for Zmake class comes from

The Zmake class and its methods are actually just automatically
invoked by the command line options.  Add some documentation about how
that happens for someone just looking at this code for the first time.

BUG=none
BRANCH=none
TEST=none

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I262d3f5bcd808944716d58e5bdf492b1657da442
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2702694
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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