eficonfig: expose append entry function

Following commits are adding support for UEFI variable management
via the eficonfig menu. Those functions needs to use
append_entry() and append_quit_entry() to construct the
menu, so move them out of their static declarations.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>