Bootfs launching performance benchmark

This test is intended to measure the performance of launching many ELF components concurrently from the bootfs. It is meant to cover the following use cases:

• Fuchsia needs to boot fast in a resource constrained environment.
• Fuchsia needs to run many ELF test components in parallel.

The test builds a custom Zircon Boot Image (ZBI) with a bootfs that contains a few synthesized ELF components. Each ELF component will report back when it hits the first line of main(). The test will measure the latency from starting component_manager to when all ELF components has reached main().