efi: Move efi_string utilities to the common directory

This file contains some useful utility functions which are not specific
to the EFI loader. Move them to lib/efi so they can be used by the app.

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

lib/efi/Makefile

@@ -9,3 +9,4 @@ obj-y += helper.o
 obj-y += load_options.o
 obj-y += memory.o
 obj-y += run.o
+obj-y += string.o

lib/efi_loader/Makefile

@@ -43,7 +43,6 @@ obj-y += efi_memory.o
 obj-y += efi_root_node.o
 obj-y += efi_runtime.o
 obj-y += efi_setup.o
-obj-y += efi_string.o
 obj-$(CONFIG_EFI_UNICODE_COLLATION_PROTOCOL2) += efi_unicode_collation.o
 obj-y += efi_var_common.o
 obj-y += efi_var_mem.o