mtd-utils: Use AC_SYS_LARGEFILE

mtd-utils: Use AC_SYS_LARGEFILE

Currently mtd-utils on 32bit systems fail on devices >2GiB due to off_t being
a signed 32bit type. Add AC_SYS_LARGEFILE to make off_t a 64bit type.
Adding AC_SYS_LARGEFILE results in _FILE_OFFSET_BITS being defined to 64
in include/config.h. To let this have an effect we must make sure that
include/config.h is included before all other includes which is
archieved by adding its inclusion to CPPFLAGS.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>