Introduce CONFIG_FIT_EXTERNAL_OFFSET

Introduce CONFIG_FIT_EXTERNAL_OFFSET to give user a choice to choose
where to put the external data.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/Kconfig b/Kconfig
index 42672a9..ee7ee30 100644
--- a/Kconfig
+++ b/Kconfig
@@ -249,6 +249,16 @@
 
 if FIT
 
+config FIT_EXTERNAL_OFFSET
+	hex "Text Base"
+	default 0x0
+	help
+	  This specifies a data offset in fit image.
+	  The offset is from data payload offset to the beginning of
+	  fit image header. When specifies a offset, specific data
+	  could be put in the hole between data payload and fit image
+	  header, such as CSF data on i.MX platform.
+
 config FIT_ENABLE_SHA256_SUPPORT
 	bool "Support SHA256 checksum of FIT image contents"
 	default y