boards: arm: kv260_r5: modify kv260 board naming

Removes the xilinx prefix on the board reference, leaving
only the board model. This also modifies the board argument
when building removing also the xilinx_ prefix from that

Instead of "-bxilinx_kv260_r5" use "kv260_r5"

Signed-off-by: Felipe Neves <felipe.neves@linaro.org>