kbuild: deb-pkg: add comment about future removal of KDEB_COMPRESS

'man dpkg-deb' describes as follows:

    DPKG_DEB_COMPRESSOR_TYPE
        Sets the compressor type to use (since dpkg 1.21.10).

        The -Z option overrides this value.

When commit 1a7f0a3 ("builddeb: allow selection of .deb compressor")
was applied, dpkg-deb did not support this environment variable.

Later, dpkg commit c10aeffc6d71 ("dpkg-deb: Add support for
DPKG_DEB_COMPRESSOR_TYPE/LEVEL") introduced support for
DPKG_DEB_COMPRESSOR_TYPE, which provides the same functionality as
KDEB_COMPRESS.

KDEB_COMPRESS is still useful for users of older dpkg versions, but I
would like to remove this redundant functionality in the future.

This commit adds comments to notify users of the planned removal and to
encourage migration to DPKG_DEB_COMPRESSOR_TYPE where possible.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: masahir0y

lib/Kconfig.debug

@@ -335,12 +335,12 @@ config DEBUG_INFO_COMPRESSED_ZLIB
 	  Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
 	  5.0+, binutils 2.26+, and zlib.
 
-	  Users of dpkg-deb via scripts/package/builddeb may find an increase in
+	  Users of dpkg-deb via debian/rules may find an increase in
 	  size of their debug .deb packages with this config set, due to the
 	  debug info being compressed with zlib, then the object files being
 	  recompressed with a different compression scheme. But this is still
-	  preferable to setting $KDEB_COMPRESS to "none" which would be even
-	  larger.
+	  preferable to setting KDEB_COMPRESS or DPKG_DEB_COMPRESSOR_TYPE to
+	  "none" which would be even larger.
 
 config DEBUG_INFO_COMPRESSED_ZSTD
 	bool "Compress debugging information with zstd"

scripts/package/debian/rules

@@ -41,6 +41,10 @@ package = $($(@:binary-%=%-package))
 # which package is being processed in the build log.
 DH_OPTIONS = -p$(package)
 
+# Note: future removal of KDEB_COMPRESS
+# dpkg-deb >= 1.21.10 supports the DPKG_DEB_COMPRESSOR_TYPE environment
+# variable, which provides the same functionality as KDEB_COMPRESS. The
+# KDEB_COMPRESS variable will be removed in the future.
 define binary
 	$(Q)dh_testdir $(DH_OPTIONS)
 	$(Q)dh_testroot $(DH_OPTIONS)