efi: Add start-up library code

When running as an EFI application, U-Boot must request memory from EFI,
and provide access to the boot services U-Boot needs.

Add library code to perform these tasks. This includes efi_main() which is
the entry point from EFI. U-Boot is built as a shared library.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

lib/Kconfig

@@ -98,4 +98,6 @@ config ERRNO_STR
 	  - if errno is null or positive number - a pointer to "Success" message
 	  - if errno is negative - a pointer to errno related message
 
+source lib/efi/Kconfig
+
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