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>

doc/uImage.FIT/source_file_format.txt

@@ -282,6 +282,9 @@ In this case the 'data' property is omitted. Instead you can use:
     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
 -----------