kbuild: make namespace.pl CROSS_COMPILE happy

Using the fixed path to /usr/bin/{nm,objdump} does not allow
CROSS_COMPILE environments to use namespace.pl. This patch causes
namespace.pl to use $NM and $OBJDUMP if defined or fall back to the nm
and objdump found in the path.

Signed-off-by: Aaron Brooks <aaron.brooks@sicortex.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>