[efi] Provide access to files stored on EFI filesystems

Provide access to local files via the "file://" URI scheme.  There are
three syntaxes:

  - An opaque URI with a relative path (e.g. "file:script.ipxe").
    This will be interpreted as a path relative to the iPXE binary.

  - A hierarchical URI with a non-network absolute path
    (e.g. "file:/boot/script.ipxe").  This will be interpreted as a
    path relative to the root of the filesystem from which the iPXE
    binary was loaded.

  - A hierarchical URI with a network path in which the authority is a
    volume label (e.g. "file://bootdisk/script.ipxe").  This will be
    interpreted as a path relative to the root of the filesystem with
    the specified volume label.

Note that the potentially desirable shell mappings (e.g. "fs0:" and
"blk0:") are concepts internal to the UEFI shell binary, and do not
seem to be exposed in any way to external executables.  The old
EFI_SHELL_PROTOCOL (which did provide access to these mappings) is no
longer installed by current versions of the UEFI shell.

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

src/config/defaults/efi.h

@@ -24,6 +24,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define TIME_EFI
 #define REBOOT_EFI
 
+#define DOWNLOAD_PROTO_FILE	/* Local filesystem access */
+
 #define	IMAGE_EFI		/* EFI image support */
 #define	IMAGE_SCRIPT		/* iPXE script image support */