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>
diff --git a/configs/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index b743c2b..cf8aedf 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -55,7 +55,7 @@
 CONFIG_CMD_RNG=y
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
-CONFIG_OF_LIST=""
+CONFIG_OF_LIST="fsl-ls1028a-kontron-sl28 fsl-ls1028a-kontron-sl28-var1 fsl-ls1028a-kontron-sl28-var2 fsl-ls1028a-kontron-sl28-var3 fsl-ls1028a-kontron-sl28-var4"
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y