* This does intentionally break source compatibility, so that a review
of concerned code is forced.
* Binary compatibility should be maintained in most cases. The values
of the constants for the writable directories are now used for the
writable system directories. The values for the non-writable
directories are mapped to "/boot/system/data/empty/...", an empty or
non-existent directory, so that they will simply be skipped in search
paths. Only code that explicitly expects to find something in a
B_COMMON_* directory, will fail.
* find_directory() and hard-coded paths use /boot/system instead of
/boot/common.
* The build system creates the writable directories in /boot/system
instead of /boot/common.
* The build system no longer installs any packages in /boot/common.
