first_stage_console: Refactor RunScript()

Introduce SpawnImage() as a reusable single-argument wrapper around
posix_spawn(), to avoid having to manually manage the child process.

Note that Bionic currently doesn't return the errno from the child's
exec() call to the caller in the parent process, which may temporarily
hide errors such as ENOENT in first_stage_console until Bionic improves.

Also, this introduces a subtle change in behavior as the first_stage.sh
script is now passed directly to the loader, which will only properly
invoke the Shell if the file contains the right shebang.

Inline the call to RunScript() to hopefully make it simpler for readers
to track the lifetime of the various processes on different code paths.

Test: run first_stage_init
Change-Id: Ifaab2be032b2080a039209295d0b5a3759764ea7