efi: stub: support running U-Boot as an EFI payload on ARM64

Implement support for launching U-Boot via an EFI stub app on ARM64.

This is more or less a straight port of the x86 implementation, but due
to the highly x86/qemu specific nature of that implementation I decided
to just split it out to its own file.

Unlike the x86 implementation, there is no debug UART here since ARM
platforms don't have a standard UART interface. However it is usually
possible to port over the debug uart implementation for you platform for
bringup purposes.

Currently this implementation doesn't provide a DTB to U-Boot and
expects U-Boot to use a built-in one, however this ought to be a fairly
trivial addition in the future.

The other significant difference to the x86 version is that rather than
copying U-Boot to CONFIG_TEXT_OFFSET, we require that U-Boot is built
position independent and copy it to EFI allocated memory.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

include/efi.h

@@ -547,7 +547,7 @@ struct efi_tpm_plat {
 extern char image_base[];
 
 /* Start and end of U-Boot image (for payload) */
-extern char _binary_u_boot_bin_start[], _binary_u_boot_bin_end[];
+extern char _binary_u_boot_bin_start[], _binary_u_boot_bin_end[], _binary_u_boot_bin_size[];
 
 /*
  * Variable Attributes