Prepare scripts for use with separate manifest repo, allowing full
build environment to be initialized with repo tool without need to
first clone meta-boot2qt.

Mirroring is now removed, as it makes more sense to use it with the
manifest repo as it then mirrors also meta-boot2qt repo.

The bblayers.conf and local.conf samples are now searched from
'sources/templates' directory, which is setup to point to either
meta-boot2qt or any boot2qt bsp layer that might be used. Same setup
is also done in the manifest repo without need to run the
b2qt-init-build-env script. This also allows setup-environment.sh
to work without the --bsp argument.

Change-Id: I9b8bfaf5d39ca2cda4cff65f196338ac96a6ba31
Reviewed-by: Timo Aarnipuro <timo.aarnipuro@qt.io>
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>