cmd: bootefi: carve out bootmgr code from do_bootefi()

This is a preparatory patch for reworking do_bootefi() in later patch.
do_bootmgr_exec() is renamed to do_efibootmgr() as we put all the necessary
code into this function.

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