Gitlab: Add a "sage-lab" stage to access a board farm

Gitlab: Add a "sage-lab" stage to access a board farm

This is the Gitlab side of adding support for the board lab connected to
the "konsulko-sage" runner. On the software side, this lab uses only
upstream labgrid. On the hardware side, each device under test is
connected to its own exporter (typically a Raspberry Pi 4) that must be
turned on (and cleanly turned off) as part of a given test cycle.

Add support for testing on a SolidRun Hummingboard 2 (imx6), Raspberry
Pi 3 and Raspberry Pi 4. In all cases, we enable additional options to
run more tests on the board. As we have some networking tests, we test
both the legacy network stack and lwIP. In the case of Pi platforms, we
test all of 32bit configuration, plain configuration and rpi_arm64, and
again with and without lwIP.

Signed-off-by: Tom Rini <trini@konsulko.com>