board: sl28: use fit image generator

Simplify the binman config and fdt nodes by using the "@..-SEQ"
substitutions and CONFIG_OF_LIST.

Signed-off-by: Michael Walle <michael@walle.cc>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
2 files changed