ublimage: NAND block size isn't set at build-time

mkimage's ublimage support can't depend of build-time board configs;
instead, this should be set in ublimage.cfg.  Since currently no configs
in u-boot override the NAND block size, hardcode it as such in
ublimage.h to fix a build failure with "make tools":
gcc [...] -o ublimage.o ublimage.c -c
In file included from ublimage.c:37:0:
ublimage.h:31:20: fatal error: config.h: No such file or directory

Cc: Heiko Schocher <hs@denx.de>
Cc: patches@linaro.org
Signed-off-by: Loïc Minier <loic.minier@linaro.org>
Acked-by: Heiko Schocher <hs@denx.de>