efi/libstub: Rewrite file I/O routine

The file I/O routine that is used to load initrd or dtb files from
the EFI system partition suffers from a few issues:
- it converts the u8[] command line back to a UTF-16 string, which is
  pointless since we only handle initrd or dtb arguments provided via
  the loaded image protocol anyway, which is where we got the UTF-16[]
  command line from in the first place when booting via the PE entry
  point,
- in the far majority of cases, only a single initrd= option is present,
  but it optimizes for multiple options, by going over the command line
  twice, allocating heap buffers for dynamically sized arrays, etc.
- the coding style is hard to follow, with few comments, and all logic
  including string parsing etc all combined in a single routine.

Let's fix this by rewriting most of it, based on the idea that in the
case of multiple initrds, we can just allocate a new, bigger buffer
and copy over the data before freeing the old one.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/x86-stub.c

@@ -421,18 +421,14 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
 	if (status != EFI_SUCCESS)
 		goto fail2;
 
-	status = handle_cmdline_files(image,
-				      (char *)(unsigned long)hdr->cmd_line_ptr,
-				      "initrd=", hdr->initrd_addr_max,
-				      &ramdisk_addr, &ramdisk_size);
+	status = efi_load_initrd(image, &ramdisk_addr, &ramdisk_size,
+				 hdr->initrd_addr_max);
 
 	if (status != EFI_SUCCESS &&
 	    hdr->xloadflags & XLF_CAN_BE_LOADED_ABOVE_4G) {
 		efi_printk("Trying to load files to higher address\n");
-		status = handle_cmdline_files(image,
-				      (char *)(unsigned long)hdr->cmd_line_ptr,
-				      "initrd=", -1UL,
-				      &ramdisk_addr, &ramdisk_size);
+		status = efi_load_initrd(image, &ramdisk_addr, &ramdisk_size,
+					 ULONG_MAX);
 	}
 
 	if (status != EFI_SUCCESS)