buildroot

target/u-boot: add 2009.11 and update target mkimage so it builds with 2009.11

In 2009.11, a number of extra files are needed to build mkimage. Adjust
the makefile to add those if available, so it builds with both 2009.11
and older releases.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>