u-boot: drop old 2009.xx versions

We use 'make tools' to build a mkimage for the host if needed by the
kernel (uImage) without having u-boot configured, but that only works
since the 2010.03 release:

http://git.denx.de/?p=u-boot.git;a=commit;h=c7c0d542a1990



So drop the old 2009.xx versions rather than having the build break
for people.

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