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>