Make static libc and guest-init functionality optional.

If one typically only boots full disk-images, one wouldn't necessaraly
want to statically link glibc, for the guest-init feature of the
kvmtool. As statically linked glibc triggers haevy security
maintainance.

Signed-off-by: Dimitri John Ledkov <dimitri.j.ledkov@intel.com>
[will: moved all the guest_init handling into builtin_setup.c]
Signed-off-by: Will Deacon <will.deacon@arm.com>