package: uboot-qoriq: fix T4240RDB u-boot selection

package: uboot-qoriq: fix T4240RDB u-boot selection

Mark T4240RDB u-boot variants as device-built and avoid installing them into rootfs.

Without this buildbot crashes during package install with:

ERROR: unable to select packages:

  u-boot-fsl_T4240RDB-nor (no such package):

    required by: world[u-boot-fsl_T4240RDB-nor]

  u-boot-fsl_T4240RDB-sdboot (no such package):

    required by: world[u-boot-fsl_T4240RDB-sdboot]

Fixes: c5d3d5fe28f7 ("package: u-boot: initial support for qoriq arch")
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21514
Signed-off-by: Robert Marko <robimarko@gmail.com>