kbuild: deb-pkg: allow to run debian/rules from output directory

'make O=... deb-pkg' creates the debian directory in the output
directory. However, currently it is impossible to run debian/rules
created in the separate output directory.

This commit delays the $(srctree) expansion by escaping '$' and by
quoting the entire command, making it possible to run debian/rules in
the output directory.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>