env: correctly handle env_load_prio

env: correctly handle env_load_prio

Only update gd->env_load_prio in generic function env_load()
and no more in the weak function env_get_location() which is
called in many place (for example in env_driver_lookup, even
for ENVOP_SAVE operation).

This patch is a preliminary step to use env_driver_lookup()/
env_get_location() in new function env_select() without
updating gd->env_load_prio.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>