Config.in: introduce the BR2_BROKEN hidden symbol

Commit [1]
"toolchain/toolchain-external/toolchain-external-synopsys-arc: mark
as broken" disabled BR2_TOOLCHAIN_EXTERNAL_SYNOPSYS_ARC by adding a
depends on BR2_BROKEN. This symbol does not exist and generates
check-symbol failures, see [2].

This commit fixes this issue by introducing the BR2_BROKEN hidden symbol
that will be used to track those known to be broken features. This
symbol will help to track features that were broken for too long and
remove them.

Fixes: [2]

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/11a8cdd2bbbd0ef4adf600e4792d75f6f2122ec8
[2] https://gitlab.com/buildroot.org/buildroot/-/jobs/8840476511

Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

Config.in

@@ -6,6 +6,15 @@ config BR2_HAVE_DOT_CONFIG
 	bool
 	default y
 
+# Hidden symbol kept to false, to temporarily mark a configuration
+# known to be broken (by depending on it). Ideally, we don't want to
+# keep broken parts for too long. The intended use of this symbol is
+# to give some time to a developer to fix the feature. Features marked
+# as broken will be removed if they are not fixed in a reasonable
+# amount of time.
+config BR2_BROKEN
+	bool
+
 config BR2_VERSION
 	string
 	option env="BR2_VERSION_FULL"