rules.mk: replace GNU specific \w with POSIX equivalant

If sed does not support the GNU \w regex extension, build attempts
lead to circular dependency warnings and finally build failure
(crc32.c not found). Build output before and after the patch on
FreeBSD is at:
http://lists.denx.de/pipermail/u-boot/2011-June/095235.html



Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>