x86: efi: Refactor the directory of EFI app and payload support

At present the EFI application and payload support codes in the x86
directory is distributed in a hybrid way. For example, the Kconfig
options for both app and payload are in arch/x86/lib/efi/Kconfig,
but the source codes in the same directory get built only for
CONFIG_EFI_STUB.

This refactors the codes by consolidating all the EFI support codes
into arch/x86/cpu/efi, just like other x86 targets.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.u-boot_on_efi b/doc/README.u-boot_on_efi
index d5903c7..0349c56 100644
--- a/doc/README.u-boot_on_efi
+++ b/doc/README.u-boot_on_efi
@@ -235,12 +235,8 @@
 lib/efi
 	payload stub, application, support code. Mostly arch-neutral
 
-arch/x86/lib/efi
-	helper functions for the fake DRAM init, etc. These can be used by
-	any board that runs as a payload.
-
 arch/x86/cpu/efi
-	x86 support code for running as an EFI application
+	x86 support code for running as an EFI application and payload
 
 board/efi/efi-x86/efi.c
 	x86 board code for running as an EFI application