fix compiling failure: execvp: /bin/bash: Argument list too long

Use GNU Make 4.2.1(such as fedora-29) to build k8s in a long directory,
it failed with `execvp: /bin/bash: Argument list too long'
[snip]
$ cd /buildarea1/hjia/wrlinux-1019/I_/suspect_/that_/if_/you_/create_/your_/project_/in_/a_/very_/deep_/directory/build_master-wr_qemux86-64_faw_2019090509/build/tmp-glibc/work/core2-64-wrs-linux/kubernetes/v1.16.0-alpha+git7054e3ead7e1a00ca6ac3ec47ea355b76061a35a-r0/kubernetes-v1.16.0-alpha+git7054e3ead7e1a00ca6ac3ec47ea355b76061a35a/src/import
$ make cross KUBE_BUILD_PLATFORMS=linux/amd64 GOLDFLAGS=""
|+++ [0804 16:38:32] Building go targets for linux/amd64:
|    ./vendor/k8s.io/code-generator/cmd/deepcopy-gen
|make[1]: execvp: /bin/bash: Argument list too long
|make[1]: *** [Makefile.generated_files:184: pkg/kubectl/cmd/testing/zz_generated.deepcopy.go] Error 127
|make: *** [Makefile:557: generated_files] Error 2
...
[snip]

From make manual [1]
$?
  The names of all the prerequisites that are newer than the target, with spaces between them.

While two `$?' was passed to bash in a line, it caused above failure,
drop a duplicated one could workaround the issue.

[1] https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html

Signed-off-by: Hongxu Jia <[email protected]>

Author: hongxu-jia

build/root/Makefile.generated_files

@@ -182,7 +182,7 @@ $(META_DIR)/$(DEEPCOPY_GEN).todo: $(DEEPCOPY_FILES)
 
 $(DEEPCOPY_FILES): $(DEEPCOPY_GEN)
 	if [[ "$(DBG_CODEGEN)" == 1 ]]; then        \
-	    echo "DBG: deepcopy needed $(@D): $?";  \
+	    echo "DBG: deepcopy needed $(@D):";  \
 	    ls -lf --full-time $@ $? || true;       \
 	fi
 	echo $(PRJ_SRC_PATH)/$(@D) >> $(META_DIR)/$(DEEPCOPY_GEN).todo
@@ -276,7 +276,7 @@ $(META_DIR)/$(DEFAULTER_GEN).todo: $(DEFAULTER_FILES)
 
 $(DEFAULTER_FILES): $(DEFAULTER_GEN)
 	if [[ "$(DBG_CODEGEN)" == 1 ]]; then         \
-	    echo "DBG: defaulter needed $(@D): $?";  \
+	    echo "DBG: defaulter needed $(@D):";  \
 	    ls -lf --full-time $@ $? || true;        \
 	fi
 	echo $(PRJ_SRC_PATH)/$(@D) >> $(META_DIR)/$(DEFAULTER_GEN).todo
@@ -380,7 +380,7 @@ $(META_DIR)/$(CONVERSION_GEN).todo: $(CONVERSION_FILES)
 
 $(CONVERSION_FILES): $(CONVERSION_GEN)
 	if [[ "$(DBG_CODEGEN)" == 1 ]]; then          \
-	    echo "DBG: conversion needed $(@D): $?";  \
+	    echo "DBG: conversion needed $(@D):";  \
 	    ls -lf --full-time $@ $? || true;         \
 	fi
 	echo $(PRJ_SRC_PATH)/$(@D) >> $(META_DIR)/$(CONVERSION_GEN).todo