image_signing: Fix loem.ini pattern for unibuilds

The grep pattern was too lenient and allowed for matches with the same
shared root key id.  E.g. NASHER also matched NASHER360

This changes the pattern to match exactly to the end of the line.

BUG=b:65284008
TEST=grep -E "[0-9]+ = NASHER$" ~/tmp/loem.ini with actual loem.ini
that will exist on the signers
BRANCH=None

Change-Id: I80a870cd512825d30c7a39e4ac6f3cffc9ea808d
Reviewed-on: https://chromium-review.googlesource.com/647800
Commit-Ready: C Shapiro <shapiroc@google.com>
Tested-by: C Shapiro <shapiroc@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: YH Lin <yueherngl@chromium.org>

scripts/image_signing/sign_official_build.sh

@@ -585,7 +585,7 @@ resign_firmware_payload() {
         # This is what updater4.sh currently uses to make the decision.
         if [[ -e "${KEY_DIR}/loem.ini" ]]; then
           # loem.ini has the format KEY_ID_VALUE = KEY_INDEX
-          local match="$(grep -E "[0-9]+ = ${key_id}" "${KEY_DIR}/loem.ini")"
+          local match="$(grep -E "[0-9]+ = ${key_id}$" "${KEY_DIR}/loem.ini")"
           local key_index="$(echo "${match}" | cut -d ' ' -f 1)"
           info "Detected key index from loem.ini as ${key_index} for ${key_id}"
           if [[ -z "${key_index}" ]]; then