sign_official_build: retain /boot for EFI devices
sign_official_build: retain /boot for EFI devices

TEST=Ran sign_official_build locally on image w/ cros_efi and checked
the signed image had /boot in rootfs.
BUG=chromium:604967
BRANCH=None

Change-Id: Id6e1a6409e07fa37a5c116c66ac937dd9aec1481
Reviewed-on: https://chromium-review.googlesource.com/335469
Commit-Ready: Amey Deshpande <62824f2effac1e9cdf0924b4b6823fc05b5ab6be@google.com>
Tested-by: Amey Deshpande <62824f2effac1e9cdf0924b4b6823fc05b5ab6be@google.com>
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot