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

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

Efi_selftest code is unusual in terms of execution path in do_bootefi(),
which make that function complicated and hard to understand. With this
patch, all efi_selftest related code will be put in a separate function.

The change also includes expanding efi_run_prepare() and efi_run_finish()
in do_bootefi_exec().

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