iMX28: Add u-boot.sb target to Makefile

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
diff --git a/.gitignore b/.gitignore
index 70a11f7..ff4bae0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,6 +36,7 @@
 /u-boot.lds
 /u-boot.ubl
 /u-boot.dtb
+/u-boot.sb
 
 #
 # Generated files