uboot-imx

envcrc: check return value of fwrite()

Newer toolchains will often complain about unchecked fwrite():
	envcrc.c:117: warning: ignoring return value of `fwrite�, declared
		with attribute warn_unused_result

So check the return value to silence the warnings.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>