gcc: fix xtensa overlay application

gcc build scripts use wrong variable name to specify xtensa overlay
application command. As a result gcc is built with the default overlay,
which leads to obscure failures later in the build process.

xtensa toolchain needs an additional configuration for a specific core
variant we're building for. This configuration is called 'overlay' and
is an archive with files for binutils, gcc and gdb that replace
corresponding files in toolchain components.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>