lib/bbconfigbuild/configfragments: disable the previous builtin fragment when enablin...

lib/bbconfigbuild/configfragments: disable the previous builtin fragment when enabling a new one

There was a flaw in the logic that allowed multiple builtin fragments with
the same prefix to be enabled at the same time. The correct behaviour
is that only one of them should be enabled, and when enabling it
all previously enabled fragments should be removed.

The issues that this caused are further explained in
https://bugzilla.yoctoproject.org/show_bug.cgi?id=15987

[YOCTO #15987]

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>