efi: Add support for loading U-Boot through an EFI stub

It is useful to be able to load U-Boot onto a board even if is it already
running EFI. This can allow access to the U-Boot command interface, flexible
booting options and easier development.

The easiest way to do this is to build U-Boot as a binary blob and have an
EFI stub copy it into RAM. Add support for this feature, targeting 32-bit
initially.

Also add a way to detect when U-Boot has been loaded via a stub. This goes
in common.h since it needs to be widely available so that we avoid redoing
initialisation that should be skipped.

Signed-off-by: Simon Glass <sjg@chromium.org>
Improvements to how the payload is built:
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

include/efi.h

@@ -268,11 +268,15 @@ struct efi_priv {
 /* Base address of the EFI image */
 extern char image_base[];
 
+/* Start and end of U-Boot image (for payload) */
+extern char _binary_u_boot_dtb_bin_start[], _binary_u_boot_dtb_bin_end[];
+
 /**
  * efi_get_sys_table() - Get access to the main EFI system table
  *
  * @return pointer to EFI system table
  */
+
 struct efi_system_table *efi_get_sys_table(void);
 
 /**