Merge the "default" and "default-by-variant" attributes. Use an
"enabled-by" expression to select the default value based on the enabled
set. This makes it possible to select default values depending on other
options. For example you could choose memory settings based on whether
RTEMS_SMP is enabled or disabled.
The change was tested by comparing the output of
./waf bspdefaults
before and after the change.
