warp7: Add Kconfig WARP7_ROOT_PART

Adding CONFIG_WARP7_ROOT_PART allows a defconfig to specify which partition
is use as the root partition on WaRP7, this is a desirable change in order
to support a different partitioning schemes. The default is the current
partition #2.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>