Changes makefile target ta_dev_kit to build the exported TA devkit even
when CFG_WITH_USER_TA is disabled. This prevents integration issues when
a unique TA devkit is generated for several platform variants/flavors
where some do support user TAs and other don't.

Change-Id: I50014c4f6ccd2e570dd52a794bede5a10c845cfe
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/optee/optee_os/+/298276
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
Tested-by: Etienne CARRIERE <etienne.carriere@foss.st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@foss.st.com>
Reviewed-by: Gatien CHEVALLIER <gatien.chevallier@st.com>
Domain-Review: Lionel DEBIEVE <lionel.debieve@foss.st.com>