Improve the source package support in case the dpkg-buildpackage is
directly used to build binary packages.

For cross-compiling, you can set CROSS_COMPILE via the environment
variable, but it is better to set it automatically - set it to
${DEB_HOST_GNU_TYPE}- if we are cross-compiling but not from the top
Makefile.

The generated source package may be carried to a different build
environment, which may have a different compiler installed.
Run olddefconfig first to set new CONFIG options to their default
values without prompting.

Take KERNELRELEASE and KBUILD_BUILD_VERSION from the version field of
debian/changelog in case it is updated afterwards.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>