u-boot 2011.03: fix mkimage build failure when u-boot isn't configured

When mkimage is built without the rest of u-boot (for kernel uImage
support), u-boot doesn't get configured for a specific board, so config.h
isn't created and the build breaks.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>