cmd: bootefi: carve out fdt handling from do_bootefi()

This is a preparatory patch for reworking do_bootefi() in later patch.

Carve out a function to handle the installation of the device tree
as a configuration table in system table.

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