Add rtems_interrupt_entry_remove(). Split up irq-generic.c into several files.
In particular, place all functions which use dynamic memory into their own
file.
Add optional macros to let the BSP customize the vector installation after
installing the first entry and the vector removal before removing the last
entry:
* bsp_interrupt_vector_install()
* bsp_interrupt_vector_remove()
Use these new customization options in the m68k/genmcf548x BSP so re-use the
generic interrupt controller support.
Update #3269.
