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

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>