bash: always prefix readline

Always prefix the readline directory otherwise autodetection may pick up
the host readline when headers are available and target == host (or are
link-compatible). Fixes:
http://autobuild.buildroot.org/results/44a/44a962623be757143a3d6d37fc6cd14964094f79/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>