tcl/board: fix changed target config filenames

tcl/board: fix changed target config filenames

In commit a1ce28b118e7 ("rename some target scripts to be
consistent with the rest") the following renames was applied, but
the old names are still referenced:
	tcl/target/{sam7se512.cfg => at91sam7se512.cfg}
	tcl/target/{sam7x256.cfg => at91sam7x256.cfg}

Fix the board files to use to correct target config filename.

Change-Id: I7698aa0da7db95c2bd9ba7ab8c260905a975c857
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5888
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins