machine-info: rename VENDOR=/MODEL= → HARDWARE_VENDOR=/HARDWARE_MODEL=

machine-info: rename VENDOR=/MODEL= → HARDWARE_VENDOR=/HARDWARE_MODEL=

Let's be more precise here. Otherwise people might think this describes
the software system or so. We already expose this via hostnamed as
HardwareVendor/HardwareModel hence use the exact same wording.

(Note that the relevant props on the dmi device are just VENDOR/MODEL,
but that's OK given that DMI really is about hardware anyway,
unconditionally, hence no chance of confusion there.)

Follow-up for 4fc7e4f374bf4401330e90e267227267abf1dcac