package/bash: don't use append-assignment where unneeded

For the first, unconditional assignment, there is no reason to use
append-assignment; it can even be misleading.

Fixes: b367ca32e5

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 1c61d9c12b)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

package/bash/bash.mk

@@ -17,7 +17,7 @@ BASH_CONF_OPTS = \
 	--with-installed-readline \
 	--without-bash-malloc
 
-BASH_CONF_ENV += \
+BASH_CONF_ENV = \
 	ac_cv_rl_prefix="$(STAGING_DIR)" \
 	ac_cv_rl_version="$(READLINE_VERSION)" \
 	bash_cv_getcwd_malloc=yes \