efi_loader: add comments on efi_file_from_path() regarding alignment

UEFI specification requires pointers that are passed to protocol member
functions to be aligned.  There's a u16_strdup in that function which
doesn't make sense otherwise  Add a comment so no one removes it
accidentally

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>