bb/ui: Use getSetVariable command for BB_CONSOLELOG

bb/ui: Use getSetVariable command for BB_CONSOLELOG

Metadata can define BB_CONSOLELOG as containing ${DATETIME} and
this can get expanded to a different value each time the variable
is read. In the case of BB_CONSOLELOG, this behaviour is not
desireable.

The values of DATE/TIME are locked down at build time but this is too
late for the purposes of ensuring the system can figure out the real
value of BB_CONSOLELOG.

The best way to do this is to set the variable into the datastore, thereby
preserving its value.

[YOCTO #8411]

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