efi/boot: rename ConfigEntry to BootEntry

efi/boot: rename ConfigEntry to BootEntry

Some of the entries are really configured, but we also have a bunch
of automatic entries. Calling them "config entries" is misleading, let's
use the more natural "boot entry".

While at it, rename:
  config_load_entries() → config_load_type1_entries()
  config_entry_add_unified() → config_load_type2_entries()
  config_title_generate() → generate_boot_entry_titles()
  config_entry_add_<type>() → config_add_entry_<type>()

No functional change.