base.bbclass: Ensure finalised data is displayed in build banner

base.bbclass: Ensure finalised data is displayed in build banner

The build banner displayed at the start of builds can be misleading since
the data store has not been finalised. As easy way to illustrate this is
to use something like:

DEFAULTTUNE = "i586"
DEFAULTTUNE_<machineoverride> = "core2"

and the banner will display the i586 tune yet the core2 tune will be
used. We can avoid this if we finalise a copy of the data before
displaying it.

[YOCTO #4225]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>