config: allow libc selection in backend mode

In backend mode, allow the upper-layer build system to force the
C library selection.

config/backend.in

@@ -16,3 +16,7 @@ config BACKEND_ARCH
 config BACKEND_KERNEL
     string
     option env="CT_BACKEND_KERNEL"
+
+config BACKEND_LIBC
+    string
+    option env="CT_BACKEND_LIBC"

config/config.mk

@@ -164,7 +164,7 @@ config.gen/cc.in: $(CC_CONFIG_FILES)
 	$(call build_gen_choice_in,$@,C compiler,CC,config/cc,,$(CCS))
 
 config.gen/libc.in: $(LIBC_CONFIG_FILES)
-	$(call build_gen_choice_in,$@,C library,LIBC,config/libc,,$(LIBCS))
+	$(call build_gen_choice_in,$@,C library,LIBC,config/libc,Y,$(LIBCS))
 
 config.gen/debug.in: $(DEBUG_CONFIG_FILES)
 	$(call build_gen_menu_in,$@,Debug,DEBUG,config/debug,$(DEBUGS))