Move creation of BootImage object to after creation of boot image objects

Due to bugs in old VMs we create the BootImage early during writing so that the arrays are more likely to be allocated. This is almost certainly not necessary now since we require at least 5.0 VMs to build. By creating the BootImage after creating the boot image objects we can save 77MB whilst creation of the boot image objects occurs. This will reduce the memory pressure of boot image writing as we won't have the compiler's IR and the boot image objects in memory at the same time.