u-boot-imx: Fix FILESEXTRAPATHS_prepend

Fixes:
  WARNING: FILESEXTRAPATHS-variable, must always use _prepend (or _append)
           type of assignment, and don't forget the colon.

Signed-off-by: Jacob Kroon <jacob.kroon@mikrodidakt.se>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>