atf: use var RCW_FOLDER to allow selecting rcw folder
Generally, rcw are deployed to ${DEPLOY_DIR_IMAGE}/rcw/${MACHINE}/.
But there are some machines that don't follow this rule. For example,
lx2160ardb deployed RCW in folder lx2160ardb_rev2 for boards with
Rev2 processor, but folder lx2160ardb for boards with Rev1 processor.
As there are only difference on RCW for them, use variable RCW_FOLDER
to allow selecting rcw folder for building atf targets. For exmaple,
to build atf which supports lx2160ardb with Rev2 processor:
RCW_FOLDER_lx2160ardb = "lx2160ardb_rev2"
