Add self-header feature for external bootloader verification #618

Job	Run time
test_external_libs (Unit tests, true)	22s
test_external_libs (external wolfHSM, config/examples/sim-wolfHSM-client-ecc.config)	31s
test_external_libs (External wolfTPM, config/examples/sim-tpm.config, true)	25s
test_external_libs (External wolfSSL, config/examples/sim.config)	21s
	1m 39s

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