make sure file name case of at91sam3uxx matches what other files include

Before this patch, at91sam3u4c.cfg includes "at91sam3uxx.cfg" which
doesn't exist - the filename was at91sam3uXX.

However, many operating systems have case sensitive file names!

Change-Id: I8b2f987f1f4214269b80ef5cba8177ce05ad90b6
Signed-off-by: Harald Welte <laforge@gnumonks.org>
Reviewed-on: http://openocd.zylin.com/247
Tested-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>