contrib/loaders: Add umbrella Makefile

contrib/loaders: Add umbrella Makefile

Add a Makefile that orchestrates the other loader Makefiles.
It assumes that the clean target can be run without cross toolchain.

at91sam7x does not successfully build and is not really needed either,
therefore left out.

Add an exception to .gitignore for any contrib/loaders Makefile.

Change-Id: I74456b768472f3190a1721bcf41a777bb8daf973
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3504
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>