add --custom-config-file-gem5 option to build gem5 kernels more easily

Author: cirosantilli

README.adoc

@@ -3495,7 +3495,7 @@ git -C "$(./getvar linux_source_dir)" fetch https://gem5.googlesource.com/arm/li
 git -C "$(./getvar linux_source_dir)" checkout gem5/v4.15
 ./build-linux \
   --arch arm \
-  --custom-config-file "$(./getvar linux_source_dir)/arch/arm/configs/gem5_defconfig" \
+  --custom-config-file-gem5 \
   --linux-build-id gem5-v4.15 \
 ;
 git -C "$(./getvar linux_source_dir)" checkout -
@@ -3571,7 +3571,7 @@ git -C "$(./getvar linux_source_dir)" checkout gem5/v4.15
 ./build-linux \
   --arch aarch64 \
   --config-fragment linux_config/display \
-  --custom-config-file "$(./getvar linux_source_dir)/arch/arm64/configs/gem5_defconfig" \
+  --custom-config-file-gem5 \
   --linux-build-id gem5-v4.15 \
 ;
 git -C "$(./getvar linux_source_dir)" checkout -

build-linux

@@ -28,6 +28,13 @@ Pass multiple times to use multiple fragment files.
 Ignore all default kernel configurations and use this file instead.
 Still uses options explicitly passed with `--config` and
 `--config-fragment` on top of it.
+'''
+        )
+        parser.add_argument(
+            '--custom-config-file-gem5', default=False, action='store_true',
+            help='''\
+Use the gem5 Linux kernel fork config as the custom config file.
+Ignore --custom-config-file.
 '''
         )
         parser.add_argument(
@@ -77,10 +84,14 @@ Configure the kernel, but don't build it.
             'CC={}'.format(cc), common.Newline,
             'O={}'.format(build_dir), common.Newline,
         ] + verbose
-        if args.custom_config_file is not None:
-            if not os.path.exists(args.custom_config_file):
+        if args.custom_config_file_gem5:
+            custom_config_file = os.path.join(common.linux_source_dir, 'arch', common.linux_arch, 'configs', 'gem5_defconfig')
+        else:
+            custom_config_file = args.custom_config_file
+        if custom_config_file is not None:
+            if not os.path.exists(custom_config_file):
                 raise Exception('config fragment file does not exist: {}'.format(args.custom_config_file))
-            base_config_file = args.custom_config_file
+            base_config_file = custom_config_file
             config_fragments = []
         else:
             base_config_file = os.path.join(common.linux_config_dir, 'buildroot-{}'.format(args.arch))