cmd: bootefi: carve out do_bootefi_image() from do_bootefi()

This is a preparatory patch for reworking do_bootefi() in later patch.
All the non-boot-manager-based (that is, bootefi <addr>) code is put
into one function, do_bootefi_image().

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>