fix compile warning for env tools

fix compile warning for env tools

Patch fixes this issue:

fw_env.c: In function ‘fw_setenv’:
fw_env.c:492:5: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]
fw_env.c: In function ‘flash_write_buf’:
fw_env.c:806:6: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]

Signed-off-by: Luka Perkov <lists@lukaperkov.net>
Acked-by: Mike Frysinger <vapier@gentoo.org>
[sw0312.kim: backport mainline commit 8603b69b8b04 to remove gcc 9 build warning]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I333ce3a5bf4caf3eed4272cf79b837eab48776a0