uboot-imx

ubifs: remove useless code

By checking ubifs source code, s_instances parameter is not
used anymore. So, set this parameter and the associated source
code under __UBOOT__ compilation.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>