mkimage: fit: spl: Add an optional static offset for external data

When building a FIT with external data (-E), U-Boot proper may require
absolute positioning for executing the external firmware. To acheive this
use the (-p) switch, which will replace the amended 'data-offset' with
'data-position' indicating the absolute position of external data.

It is considered an error if the requested absolute position overlaps with the
initial data required for the compact FIT.

Signed-off-by: Teddy Reed <teddy.reed@gmail.com>
diff --git a/doc/mkimage.1 b/doc/mkimage.1
index 4b3a255..ffa7d60 100644
--- a/doc/mkimage.1
+++ b/doc/mkimage.1
@@ -152,6 +152,12 @@
 CONFIG_OF_CONTROL in U-Boot.
 
 .TP
+.BI "\-p [" "external position" "]"
+Place external data at a static external position. See \-E. Instead of writing
+a 'data-offset' property defining the offset from the end of the FIT, \-p will
+use 'data-position' as the absolute position from the base of the FIT.
+
+.TP
 .BI "\-r
 Specifies that keys used to sign the FIT are required. This means that they
 must be verified for the image to boot. Without this option, the verification
diff --git a/doc/uImage.FIT/source_file_format.txt b/doc/uImage.FIT/source_file_format.txt
index 3f54180..91aa89a 100644
--- a/doc/uImage.FIT/source_file_format.txt
+++ b/doc/uImage.FIT/source_file_format.txt
@@ -282,6 +282,9 @@
     aligned to a 4-byte boundary.
   - data-size : size of the data in bytes
 
+The 'data-offset' property can be substituted with 'data-position', which
+defines an absolute position or address as the offset. This is helpful when
+booting U-Boot proper before performing relocation.
 
 9) Examples
 -----------