kbuild: deb-pkg: set CROSS_COMPILE only when undefined

Commit 3ab18a62 ("kbuild: deb-pkg: improve the usability of source
package") set needless CROSS_COMPILE.

For example, 'make allnoconfig bindeb-pkg' on a x86_64 system will set
CROSS_COMPILE=i686-linux-gnu-, where the biarch compiler 'gcc' should
work for building the i386 kernel.

  $ uname -m
  x86_64
  $ make allnoconfig bindeb-pkg >/dev/null
  dpkg-architecture: warning: specified GNU system type i686-linux-gnu does not match CC system type x86_64-linux-gnu, try setting a correct CC environment variable
   dpkg-source --before-build .
   debian/rules binary
  scripts/Kconfig.include:39: C compiler 'i686-linux-gnu-gcc' not found
  make[6]: *** [scripts/kconfig/Makefile:77: olddefconfig] Error 1
  make[5]: *** [Makefile:693: olddefconfig] Error 2
  make[4]: *** [Makefile:358: __build_one_by_one] Error 2
  make[3]: *** [debian/rules:7: build-arch] Error 2
  dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
  make[2]: *** [scripts/Makefile.package:127: bindeb-pkg] Error 2
  make[1]: *** [Makefile:1657: bindeb-pkg] Error 2
  make: *** [Makefile:358: __build_one_by_one] Error 2

Check whether CROSS_COMPILE is defined, instead of whether it is non-empty.

If you invoke debian/rules via Kbuild, CROSS_COMPILE is always defined
in the top Makefile.

Fixes: 3ab18a62

 ("kbuild: deb-pkg: improve the usability of source package")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>