eficonfig: expose eficonfig_create_device_path()

Following commits are adding support for UEFI variable management
via the eficonfig menu. Those functions needs to use
eficonfig_create_device_path() to construct the full device path
from device path of the volume and file path, so move it
out of their static declarations.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>