board: sl28: implement additional bootsources

The board is able to boot from the following source:
 - user-updateble SPI flash
 - write-protected part of the same SPI flash
 - eMMC
 - SD card

Implement the needed function hooks to support all of these boot
sources.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index b0b5fb1..4d50c68 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -12,6 +12,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1028a-kontron-sl28"
 CONFIG_SPL_TEXT_BASE=0x18010000
 CONFIG_SYS_FSL_SDHC_CLK_DIV=1
+CONFIG_SPL_MMC=y
 CONFIG_SPL_SERIAL=y
 CONFIG_SPL_SIZE_LIMIT=0x20000
 CONFIG_SPL_SIZE_LIMIT_PROVIDE_STACK=0x0
@@ -46,9 +47,11 @@
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x18009ff0
 CONFIG_SYS_SPL_MALLOC=y
+CONFIG_SPL_SEPARATE_BSS=y
+CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
+CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x900
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_SPI_LOAD=y
-CONFIG_SYS_SPI_U_BOOT_OFFS=0x230000
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_SYS_BOOTM_LEN=0x800000