efi_loader: add efi_create_indexed_name()

This function will be used from several places in UEFI subsystem
to generate some specific form of utf-16 variable name.
For example, L"Capsule0001"

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Move function to separate module.
Use char * as argument instead of u16 *.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>