nand_spl: fix u-boot.lst breakage

Commit 97b24d3d "common: Add symbol
handling for generic lists into Makefile" introduced build errors in many
nand_spl targets, complaining of a missing u-boot.lst.  When not doing an
out-of-tree build, $(obj) expands to nothing, so GCC ended up being given
-I with no argument (or rather, -ansi was the argument).  The failure
didn't show up during a -j1 build because it was picking up the non-SPL
version of u-boot.lst.

Signed-off-by: Scott Wood <scottwood@freescale.com>