image-fit: Fix compiling error caused by autoconf.h

Commit ec6617c3 includes autoconf.h in image-fit.c, causing conflict
for board odroid-xu3 which overwrites CONFIG_SYS_BOARD in header
file. Move the include higher and use linux/kconfig.h instead of
generated/autoconf.h.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Alison Wang <alison.wang@nxp.com>