OvmfPkg/LoongArchVirt: use PlatformSecureLibNull for secure boot builds

[20000419]

Motivation

• LoongArchVirt Secure Boot configuration selected OvmfPkg/Library/PlatformSecureLib, whose UserPhysicalPresent() unconditionally returns TRUE, enabling an in-guest privileged OS to bypass Secure Boot protections by setting CustomMode and replacing PK/KEK/db without signature checks.
• The change prevents treating all callers as physically present for Secure Boot variable flows and restores a real physical-presence gate by using the null platform secure implementation that honors PcdUserPhysicalPresence.

Description

• Update OvmfPkg/LoongArchVirt/LoongArchVirtQemu.dsc to map PlatformSecureLib to SecurityPkg/Library/PlatformSecureLibNull/PlatformSecureLibNull.inf when SECURE_BOOT_ENABLE=TRUE instead of OvmfPkg/Library/PlatformSecureLib/PlatformSecureLib.inf.
• Preserve AuthVariableLib, SecureBootVariableLib, SecureBootVariableProvisionLib, and PlatformPKProtectionLib so authenticated-variable functionality remains enabled while removing the unconditional physical-presence bypass.
• No changes were made to AuthVariableLib logic; this is a configuration-level remediation to prevent untrusted callers from implicitly satisfying physical-presence checks.

Testing

• Attempted a secure-boot build with SECURE_BOOT_ENABLE=TRUE via source edksetup.sh && build -a LOONGARCH64 -t GCC5 -b DEBUG -p OvmfPkg/LoongArchVirt/LoongArchVirtQemu.dsc -D SECURE_BOOT_ENABLE=TRUE, which failed due to the environment lacking the GCC5 toolchain ([GCC5] not defined), so a full build could not be completed here.
• Verified the intended configuration change with git diff and by inspecting OvmfPkg/LoongArchVirt/LoongArchVirtQemu.dsc, confirming PlatformSecureLib now points to PlatformSecureLibNull.
• Confirmed PlatformSecureLibNull uses mUserPhysicalPresence (default FALSE via PcdUserPhysicalPresence) by inspecting SecurityPkg/Library/PlatformSecureLibNull/PlatformSecureLibNull.c to validate the remediation's effect on the presence check.

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Codex Task

[gemini-code-assist]

Code Review

This pull request updates the LoongArchVirtQemu.dsc file to use the PlatformSecureLibNull library instead of PlatformSecureLib when SECURE_BOOT_ENABLE is set to true. I have no feedback to provide.