[release-4.18] Add an agent flag to disable installing boatloaders

[sunku5494]

Pipes the new agent flag (enable_bios_bootloader_install) to agents. This flag disables bootloader install (calls to grub-install) by default for security reasons.

Part of mitigation of CVE-2026-43003.

Related-Bug: 2148310
Change-Id: I694bbe121e09e7e0b2e6c5ab3746f7943385190a

(cherry picked from commit e38ae0c) (cherry picked from commit 9a8d91a) (cherry picked from commit 0fb6ca6) (cherry picked from commit 008be86) (cherry picked from commit b07e9d0)

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