rename KERN_END to KERN_SIZE, same for KERN_ROEND

[rottaran]

Use VIRT_END as virtual address where the kernel image ends. This is analogous to VIRT_ADDR for the start address. The kernel size can be queried via VIRT_END-VIRT_ADDR.

In PhysPtr.hh isImageAddr() the VIRT_END can be used directly.

KERN_END is defined in some files but not used. Thus it should be removed.

The init-array sections for constructor addresses in the linker script should be moved to the read-only kern segment.