.SHELLFLAGS is extended by option pipefail to make pipes fail if any pipe
element fails. As .SHELLFLAGS is exported into sub-make instances it
must be unexported before calling third-party build systems recursively.
To support device-less protocol-stacks only ports, we can use a
Virt I/O Linux kernel flavor, and export the lx_kit/lx_emul parts
not depending on platform API and devices.
Ref #4397
