efi_loader: Add device path related functions for initrd via Boot####

On the following patches we allow for an initrd path to be stored in
Boot#### variables.  Specifically we encode in the FIlePathList[] of
the EFI_LOAD_OPTIONS for each Boot#### variable.

The FilePathList[] array looks like this:
kernel - 0xff - VenMedia(initrd GUID) - initrd1 - 0x01 initrd2 - 0xff
So let's add the relevant functions to concatenate and retrieve a device
path based on a Vendor GUID.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reformat function descriptions.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>