sanitycheck fails to merge OVERLAY_CONFIG properly

[mniestroj]

Describe the bug
Sanitycheck fails to merge OVERLAY_CONF properly before passing this argument to build system.

To Reproduce
Steps to reproduce the behavior:

1. ./scripts/sanitycheck -p qemu_x86 -T samples/net/sockets/echo_client/ -i
2. Look at ./sanity-out/qemu_x86/samples/net/sockets/echo_client/sample.net.sockets.echo_client.bt/build.log
3. Look at merged configuration, there is no overlay-bt.conf that should be mentioned there:

Loaded configuration '/zephyr/boards/x86/qemu_x86/qemu_x86_defconfig'
Merged configuration '/zephyr/samples/net/sockets/echo_client/prj.conf'
Merged configuration '/zephyr/samples/net/sockets/echo_client/boards/qemu_x86.conf'
Configuration saved to '/zephyr/sanity-out/qemu_x86/samples/net/sockets/echo_client/sample.net.sockets.echo_client.bt/zephyr/.config'

Expected behavior
overlay-bt.conf should be taken into account:

Merged configuration 'overlay-bt.conf'

Impact
Sanitycheck doesn't properly test all configurations within the project, because it fails to merge OVERLAY_CONF options together.

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK
• Commit SHA or Version used: c793c0e

[nashif]

should be fixed