package/e2fsprogs: make /sbin/fsck available

systemd looks for fsck in /sbin, so make symlink:
/sbin/fsck -> /usr/sbin/fsck

[Thomas: add comment.]

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>