lx2160a: Fix distroboot device list for configs without USB/SCSI/etc

The BOOT_TARGET_DEVICES list for distro_bootcmd was hard-coded to assume
that all boot devices are available/enabled in the configuration,
thus ignoring the actual config settings. The config_distro_bootcmd.h
header file specifically has compile-time checks to detect such problems.

To allow disabling USB, SCSI, etc. in custom lx2160a board configs,
make it depend on the config settings and use only the enabled features.

Signed-off-by: Daniel Klauer <daniel.klauer@gin.de>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>