efi: Create a common efi_main() implementation for the stub

At present x86 and ARM have different implementations, but they are
similar enough that it is not too hard to unify them.

Create a new common function, with arch-specific pieces at the start
(setting up the address to which to copy U-Boot) and end (to jump to
U-Boot).

For now, nothing uses this code.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/efi.h

@@ -473,6 +473,7 @@ static inline struct efi_mem_desc *efi_get_next_mem_desc(
  *	to U-Boot
  * @info_size: Size of the info list @info in bytes
  * @next_hdr: Pointer to where to put the next header when adding to the list
+ * @jump_addr: Address to jump to to start U-Boot
  */
 struct efi_priv {
 	efi_handle_t parent_image;
@@ -496,6 +497,7 @@ struct efi_priv {
 	struct efi_info_hdr *info;
 	unsigned int info_size;
 	void *next_hdr;
+	ulong jump_addr;
 };
 
 /*