chore: Consolidate mount-generator for HostOS and GuestOS

[shilingwang]

#NODE-1412

Consolidate mount-generator for HostOS and GuestOS

The mount-generator systemd generator script was maintained as two nearly identical copies:
upgrade/systemd-generators/hostos/mount-generator
upgrade/systemd-generators/guestos/mount-generator

Both shared the same structure, boilerplate, and /boot mount logic. Only two things differed:
Boot device: HostOS uses LVM volumes (/dev/hostlvm/A_boot), GuestOS uses partition UUIDs (/dev/disk/by-partuuid/...)
/var setup: HostOS uses a plain ext4 LVM-backed mount (var-setup.service + var.mount), GuestOS uses LUKS encryption (systemd-cryptsetup@var_crypt.service)

Changes

1. Merged both scripts into a single upgrade/systemd-generators/mount-generator
2. Added two identity files (os-type-hostos, os-type-guestos) installed to /etc/ic/os-type at image build time — the merged script reads this file to branch on OS-specific behavior
3. Updated hostos.bzl and guestos.bzl to point to the shared script and install the appropriate identity file
4. Deleted the now-redundant hostos/mount-generator and guestos/mount-generator

How the identity is passed

component_files in hostos.bzl / guestos.bzl already maps source files to destination paths inside the image. The two os-type-* files are both installed to /etc/ic/os-type in their respective images, but with different content (hostos or guestos). This is the same pattern already used elsewhere (e.g. setup-ssh-user-keys-hostos.sh / setup-ssh-user-keys-guestos.sh → same destination path).