mkimage: add UBL header support for booting davinci cpus

creating an u-boot.ubl file, which contains the UBL Header
needed for booting from NAND with the RBL from TI. For more
information read doc/README.ublimage.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/.gitignore b/.gitignore
index 34a2ca9..dbf545f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,6 +33,7 @@
 /u-boot.sha1
 /u-boot.dis
 /u-boot.lds
+/u-boot.ubl
 
 #
 # Generated files