zephyr: enable CONFIG_EXTERNAL_STORAGE
zephyr: enable CONFIG_EXTERNAL_STORAGE

Enables the CONFIG_EXTERNAL_STORAGE features now that all the underlying
requirements have been met.

BRANCH=none
BUG=b:167392037
TEST=zmake testall
TEST=build volteer, flash, see it boot

Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Change-Id: Id4e59a60ac7df963e836640d6519eaba24198467
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2693528
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