The new configs/imx28_xea_sb_defconfig is introduced to facilitate
building the single binary u-boot.sb fox XEA board.

The biggest distinction from "normal" XEA imx28_xea_sb_defconfig is
support for USB mass storage devices (pen drives).

To achieve that, the CONFIG_DM_USB is enabled and supported.

Signed-off-by: Lukasz Majewski <lukma@denx.de>