boards: siemens: iot2050: Fix boot configuration

The common env bits now come via ti_armv7_common.env, include it.
Furthermore restore the board-specific boot targets and their ordering
that is now enforced k3-wide differently. Finally, enable
CONFIG_LEGACY_IMAGE_FORMAT explicitly which got lost while turning
FIT_SIGNATURE on by default for k3 devices.

Fixes: 53873974 ("include: armv7: Enable distroboot across all configs")
Fixes: 4ae1a247 ("env: Make common bootcmd across all k3 devices")
Fixes: 86fab110 ("Kconfig: Enable FIT_SIGNATURE if ARM64")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>