[efi] Add ability to extract device path from an EFI load option

[efi] Add ability to extract device path from an EFI load option

An EFI boot option (stored in a BootXXXX variable) comprises an
EFI_LOAD_OPTION structure, which includes some undefined number of EFI
device paths.  (The structure is extremely messy and awkward to parse
in C, but that's par for the course with EFI.)

Add a function to extract the first device path from an EFI load
option, along with wrapper functions to read and extract the first
device path from an EFI boot variable.

Signed-off-by: Michael Brown <mcb30@ipxe.org>