boot/optee-os: fix TA files installation

This change fixes the TA binary files install sequence that were
previously install in target filesystem though
OPTEE_OS_INSTALL_IMAGES_CMDS instead of expected
OPTEE_OS_INSTALL_TARGET_CMDS.

This change removes useless mkdir prior call to $(INSTALL) that
already handles parent directory creation when needed.

This change conditions the .ta files installation upon their
availability as they may exist or not depending on the exact OPTEE
version/configuration selected.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>