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/common.h

@@ -1021,6 +1021,13 @@ int cpu_release(int nr, int argc, char * const argv[]);
 	offsetof(struct structure, member) == offset, \
 	"`struct " #structure "` offset for `" #member "` is not " #offset)
 
+/* Avoid using CONFIG_EFI_STUB directly as we may boot from other loaders */
+#ifdef CONFIG_EFI_STUB
+#define ll_boot_init()	false
+#else
+#define ll_boot_init()	true
+#endif
+
 /* Pull in stuff for the build system */
 #ifdef DO_DEPS_ONLY
 # include <environment.h>