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/board/kontron/sl28/sl28.h b/board/kontron/sl28/sl28.h
new file mode 100644
index 0000000..7f01050
--- /dev/null
+++ b/board/kontron/sl28/sl28.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#ifndef __SL28_H
+#define __SL28_H
+
+enum boot_source {
+	BOOT_SOURCE_UNKNOWN,
+	BOOT_SOURCE_SDHC,
+	BOOT_SOURCE_MMC,
+	BOOT_SOURCE_I2C,
+	BOOT_SOURCE_SPI,
+};
+
+enum boot_source sl28_boot_source(void);
+
+#endif