Genode build system allows to easily enable 'ccache' for builds. This
change allows to enable using 'ccache' also for build of reference
Linux kernel used during porting device drivers.
To enable 'ccache' it is enough to pass value of 'CC' variable when
executing Linux build but this build by default depends on time when it
is built which causes 'ccache' misses. To solve this issue additional
flags are passed to make build independent from time, current user and
host on which build is performed.
Issue #4718
* Move common SMP and NO_HZ_IDLE variables and functions into
generic lx_emul shadow implementations, and integrate them
into the common lx_emul import rules
* Enable SMP and NO_HZ_IDLE within virt_linux kernel configuration
* Adapt pc drivers and wireguard accordingly
* Use original Linux softirq implementation in wireguard
* Remove erroneous softirq shadow implementation
Ref genodelabs/genode#4540
Ref genodelabs/genode#4562
.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
