Fix remaining incorrect reference to target/at91sam3uXX.cfg

Fix remaining incorrect reference to target/at91sam3uXX.cfg

Commit 1794e5ee5452c83b5bef6d0a5a8a3f23d647e9c6 renamed the file to
have all lowercase characters according to most references to the
file, but the commit didn't change the existing reference to the
old filename.

Change-Id: I380e52e947a8091d48cf010e3919bf2caed7fdff
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/248
Tested-by: jenkins