xtensa: don't build and run the reset handler twice

Currently Zephyr links reset-vector.S twice in xtensa builds:
into the bootloader and the main image. It is run at the end
of the boot loader execution and immediately after that again
in the beginning of the main code. This patch adds a
configuration option to select whether to link the file to the
bootloader or to the application. The default is to the
application, as needed e.g. for QEMU, SOF links it to the
bootloader like in native builds.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>