Enable veriexec for loader

This relies on libbearssl and libsecureboot
to verify files read by loader in a maner equivalent
to how mac_veriexec

Note: disabled by default.
Use is initially expected

Enable veriexec for loader

This relies on libbearssl and libsecureboot
to verify files read by loader in a maner equivalent
to how mac_veriexec

Note: disabled by default.
Use is initially expected to be by embeded vendors

Reviewed by: emaste, imp
Sponsored by: Juniper Networks
Differential Revision: D16336

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