arch/xtensa: get rid of an intermediate blind kconfig option

It is not needed to have an intermediate blind option, we can just
hide the prompt behind the same dependency as the non-blind symbol.

Update our qemu-xtensa defconfig acordingly (note: it was using
different values for both options, which is not possible; the blind
option was just set to the non-blind one in the .config).

Also remove an unneeded empty default for the BR2_XTENSA_OVERLAY_DIR
string option (strings are empty by default).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>