buildroot

uboot-tools: fix build breakage

Fix uboot-tools build breakage as reported by Peter Korsgaard.
Previously config.h was included in imximage.h but not the fw_env tool.

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