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>

package/bash/bash.mk

@@ -12,6 +12,7 @@ BASH_LICENSE = GPLv3+
 BASH_LICENSE_FILES = COPYING
 
 BASH_CONF_ENV += \
+	ac_cv_rl_prefix="$(STAGING_DIR)" \
 	ac_cv_rl_version="$(READLINE_VERSION)" \
 	bash_cv_job_control_missing=present \
 	bash_cv_sys_named_pipes=present \