efi: Rename the lib/efi directory

This directory was created when U-Boot gained the ability to run as an
EFI app in 2015. Since then the EFI-loader feature has been added.

The code in lib/efi is not actually used by the loader, so the name is
confusing.

Rename the directory to efi_client to indicate that it includes files
just for U-Boot being a client of EFI, i.e. the EFI app and stub.

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

Makefile

@@ -1891,7 +1891,7 @@ u-boot-payload.lds: $(LDSCRIPT_EFI) FORCE
 quiet_cmd_u-boot_payload ?= LD      $@
       cmd_u-boot_payload ?= $(LD) $(LDFLAGS_EFI_PAYLOAD) -o $@ \
       -T u-boot-payload.lds arch/x86/cpu/call32.o \
-      lib/efi/efi.o lib/efi/efi_stub.o u-boot.bin.o \
+      lib/efi_client/efi.o lib/efi_client/efi_stub.o u-boot.bin.o \
       $(addprefix arch/$(ARCH)/lib/,$(EFISTUB))
 
 u-boot-payload: u-boot.bin.o u-boot-payload.lds FORCE