ext: plumb out Kconfig support for external source code

Add missing Kconfig files and connections to expose Kconfig options
in ext/ directories.  Fixup QMSI to only be exposed on platforms that
utilize it.

Change-Id: I6c6c5011b2bf2966f65aa8279dc594a244821956
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>