util/update_release_branch.py: Use relative paths
util/update_release_branch.py: Use relative paths

BRANCH=none
BUG=b:179093018
TEST=none

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I0a25000e739bc2edc665824964c51822d81c96be
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2676268
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

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