Add a PLYMOUTH place to rotate-screen for the DRM panel orientation

[eliasbakken]

Problem

Companion fix to Rebuild#70 (installing Plymouth for a real boot splash). The splash showed up, but wasn't rotated - Plymouth's DRM renderer ignores fbcon=rotate: entirely, since that only affects the legacy fbcon text console.

Fix

Plymouth reads the kernel's DRM panel orientation instead, set via video=<connector>:panel_orientation=<value> on the kernel command line - a mechanism none of the other four rotate-screen places (FBCON/XORG/CMDLINE/WESTON) touch.

• Fills in the ROT_PLYMOUTH variable that was already sitting there unused in each rotation case.
• Adds update_plymouth(), mirroring update_cmdline()'s mount/sed/umount pattern against armbianEnv.txt.
• Wires PLYMOUTH into the PLACES array and dispatch.
• server.go now calls cmdRotateScreen(..., "PLYMOUTH") alongside the existing CMDLINE/XORG/WESTON calls in runInstallFinishedCommands.

Caveat: only 0->normal and 270->left_side_up are empirically confirmed on real hardware. 90->right_side_up and 180->upside_down are inferred by elimination from the four DRM panel-orientation values and flagged as such in comments - worth validating on hardware before fully trusting those two.

Test plan

• Ran the actual updated script live on real hardware (rotate-screen 270 PLYMOUTH), confirmed it correctly writes/updates video=HDMI-A-1:panel_orientation=left_side_up
• Confirmed idempotent - re-running with a different rotation updates the value in place rather than duplicating the parameter
• go build passes with the server.go change
• Confirmed end-to-end on a real reboot: splash now shows in the correct orientation

Note: go test ./... currently fails on a pre-existing, unrelated crash in TestHandleSerialCommand - confirmed present on clean main before this change, not something introduced here.

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