bootstd: Add an implementation of EFI bootmgr

bootstd: Add an implementation of EFI bootmgr

Add a bootmeth driver which handles EFI boot manager, using EFI_LOADER.

In effect, this provides the same functionality as the 'bootefi bootmgr'
command and shares the same code. But the interface into it is via a
bootmeth, so it does not require any special scripts, etc.

For now this requires the 'bootefi' command be enabled. Future work may
tidy this up so that it can be used without CONFIG_CMDLINE being enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>