zephyr: shim version.c and generate ec_version.h
zephyr: shim version.c and generate ec_version.h

This change shims common/version.c which depends on generating
ec_version.h.

Generating ec_version.h adds a new custom cmake library that will
call into a custom command. This ensures that:
(1) The generated ec_version.h is built at build time.
(2) The things that depend on ec_version.h are only recompiled
    if ec_version.h changes.

BRANCH=none
BUG=b:174873770
TEST=zmake testall
TEST=Use zmake to build volteer twice and make sure we don't rebuild
     version.c.

Change-Id: I0302390103d6ae4909cbcfba9c78b2deaaa02d58
Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2633393
Commit-Queue: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
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