make: Fix npcx compile error due to missing ecst binary
make: Fix npcx compile error due to missing ecst binary

Add an explicit dependency on utils-build (includes ecst) to prevent
ec_elf_to_flat from running before ecst is built during parallel make.

BUG=chrome-os-partner:52777
BRANCH=None
TEST=Modify Makefile.rules to add 5sec sleep in $(build-utils) rule,
verify that `make BOARD=kevin clean && make BOARD=kevin -j` still
succeeds.

Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Change-Id: Ia443f8b880cbd4451d41302817e176201f7870dc
Reviewed-on: https://chromium-review.googlesource.com/350162
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

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