test: environment in ext4

Add basic test to persistent environment in ext4:
save and load in host ext4 file 'uboot.env'.

On first execution an empty EXT4 file system is created in
persistent data dir: env.ext4.img.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Stephen Warren <swarren@nvidia.com>