layerscape: define only one package for ls-rcw

layerscape: define only one package for ls-rcw

We do not have to define package for each board, and
consider variant's building/installing.
It is easier to maintain ls-rcw with only one package
installing all boards RCW binaries as intermediate
files, each of which is just about hundreds of bytes.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>