Fix for integration tracker problem ID=884665.

Fix for integration tracker problem ID=884665.

RTLINUX_DIR variable was used inconsistently (different meaning inside
rtlinux tree compared to the general meaning in the OCERA tree).

Solution adopted, to maintain compatibility toward external developers,
the particular rtlinux meaning of the variable has been isolated in the
rtlinux tree and, more important, has been adopted a different variable
RTL_DIR that has the same meaning both inside and outside the rtlinux
tree.