efi_loader: Add OS notifications for SetVariable at runtime

Previous patches enable SetVariable at runtime using a volatile storage
backend using EFI_RUNTIME_SERVICES_DATA allocared memory. Since there's
no recommendation from the spec on how to notify the OS, add a volatile
EFI variable that contains the filename relative to the ESP. OS'es
can use that file and update it at runtime

$~ efivar -p -n b2ac5fc9-92b7-4acd-aeac-11e818c3130c-RTStorageVolatile
GUID: b2ac5fc9-92b7-4acd-aeac-11e818c3130c
Name: "RTStorageVolatile"
Attributes:
	Boot Service Access
	Runtime Service Access
Value:
00000000  75 62 6f 6f 74 65 66 69  2e 76 61 72 00           |ubootefi.var.   |

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