Change "update required" message to "update available"

Originally, rpi-eeprom-update was only for critically important
security or hardware compatibility requirements. Change the
text of rpi-eeprom-update to indicate that an update is available
rather than being required.
N.B. The systemd service runs with the -a flag which automatically
applies available updates because most users just use the release
folder which is only updated for major/critical updates.

rpi-eeprom-update

@@ -448,7 +448,7 @@ EOF
 printVersions()
 {
    if [ "${ACTION_UPDATE_BOOTLOADER}" = 1 ]; then
-      echo "BOOTLOADER: update required"
+      echo "BOOTLOADER: update available"
    else
       echo "BOOTLOADER: up-to-date"
    fi
@@ -458,7 +458,7 @@ printVersions()
    echo " FW DIR: ${FIRMWARE_IMAGE_DIR}"
 
    if [ "${ACTION_UPDATE_VL805}" = 1 ]; then
-      echo "VL805: update required"
+      echo "VL805: update available"
    else
       if [ "$(id -u)" = "0" ]; then
          echo "VL805: up-to-date"
@@ -621,7 +621,7 @@ checkVersion()
    lookupVersionInfo
 
    if [ "${BOOTLOADER_UPDATE_VERSION}" -gt "${BOOTLOADER_CURRENT_VERSION}" ]; then
-      echo "*** UPDATE REQUIRED ***"
+      echo "*** UPDATE AVAILABLE ***"
       printVersions
       write_status_info "EXIT_UPDATE_REQUIRED"
       exit ${EXIT_UPDATE_REQUIRED}