Add a simple RAM disk driver

* It uses physical pages directly, not wasting kernel address space.
* The user interface is somewhat crude, it uses writes to a control
  device (output to serial debug/syslog), e.g.
    echo register 4g > /dev/disk/virtual/ram/control
  to register a 4 GB RAM disk. It is published under
  /dev/disk/virtual/ram/<index>/raw and can be formatted with DriveSetup
  like any other disk. Unregistering is broken ATM.
* It's not on the image by default, but can be added via
    AddNewDriversToHaikuImage disk virtual : ram_disk ;
* I found it quite useful when working on large HaikuPorts ports.
  E.g. mounting a RAM disk at the port's work directory shaves a lot of
  time off disk heavy operations. Obviously one shouldn't let changes
  lie around on it without back up.

src/add-ons/kernel/drivers/disk/virtual/Jamfile

@@ -2,5 +2,6 @@ SubDir HAIKU_TOP src add-ons kernel drivers disk virtual ;
 
 #SubInclude HAIKU_TOP src add-ons kernel drivers disk virtual fmap ;
 SubInclude HAIKU_TOP src add-ons kernel drivers disk virtual nbd ;
+SubInclude HAIKU_TOP src add-ons kernel drivers disk virtual ram_disk ;
 SubInclude HAIKU_TOP src add-ons kernel drivers disk virtual remote_disk ;
 SubInclude HAIKU_TOP src add-ons kernel drivers disk virtual virtio_block ;

src/add-ons/kernel/drivers/disk/virtual/ram_disk/Jamfile

@@ -0,0 +1,8 @@
+SubDir HAIKU_TOP src add-ons kernel drivers disk virtual ram_disk ;
+
+
+SubDirHdrs $(HAIKU_TOP) src system kernel device_manager ;
+UsePrivateKernelHeaders ;
+
+
+KernelAddon ram_disk : ram_disk.cpp ;