efi_loader: catch misspelled bootefi subcommand

If 'bootefi hello' or 'bootefi selftest' can be executed depends on the
configuration.

If an invalid non-numeric 1st argument is passed to bootefi, e.g.
'bootefi hola', this string is converted to 0 and U-Boot jumps to
this typically invalid address.

With the patch the online help is shown instead.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>