lib/bbconfigbuild/configfragments: add support for listing and enabling built-in...

lib/bbconfigbuild/configfragments: add support for listing and enabling built-in fragments

Sample output:

$ bitbake-config-build enable-fragment machine/qemuarm
Fragment machine/qemuarm added to /srv/storage/alex/yocto/build-64-alt/conf/auto.conf.

$ bitbake-config-build list-fragments
Available built-in fragments:
machine/...	Sets MACHINE = ...
distro/...	Sets DISTRO = ...

Enabled built-in fragments:
machine/qemuarm	Sets MACHINE = "qemuarm"

... (standard on-disk fragments output follows)

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