managerd: adjust log level for failed backend selections

The following boot state backend's throw a critical message,
when the respective boot state backend is not selected
  - boot-state-uboot-env
  - boot-state-uboot-ext
  - boot-state-efi

Reduce the log level by using g_message instead of g_critical

Signed-off-by: Moses Christopher <BollavarapuMoses.Christopher@in.bosch.com>