Control buildsets using the --with-* command line option

The following thread has raise an important issue on how we can test newer versions of tools. This ticket is about providing a solution:

https://lists.rtems.org/pipermail/devel/2022-May/071458.html

The solution is allow users to specific a tool set on the command line for a build of tools. The standard option is --with-.*. For example a command line could be:

../source-builder/sb-set-builder \
    --prefix=/opt/rtems/6 \
    --log=arm.txt \
    --with-rtems-gcc=tools/rtems-gcc-head-newlib-head \
    6/rtems-arm

The buildset logic is to be changed to expand macros for buildset file names and to add a new buildset specific keyword %defineifnot. The tools default file becomes:

#
# Default tools configuration.
#
%include 6/rtems-base.bset

#
# Default RTEMS build. Override on the command line to the
# the RSB. For example to test the latest versino of gcc use:
#
#  --with-rtems-gcc=tools/rtems-gcc-head-newlib-head
#
%defineifnot with_rtems_dtc      devel/dtc-1.6.1-1
%defineifnot with_rtems_expat    devel/expat-2.4.8-1
%defineifnot with_rtems_gmp      devel/gmp-6.2.1
%defineifnot with_rtems_gdb      tools/rtems-gdb-11.2
%defineifnot with_rtems_binutils tools/rtems-binutils-2.38
%defineifnot with_rtems_gcc      tools/rtems-gcc-10-newlib-head
%defineifnot with_rtems_tools    tools/rtems-tools-6

#
# Build gdb first to raise the Python install error as early as
# possible.  GDB needs expat so it needs to be built before.
#
%{with_rtems_dtc}
%{with_rtems_expat}
%{with_rtems_gmp}
%{with_rtems_gdb}
%{with_rtems_binutils}
%{with_rtems_gcc}
%{with_rtems_tools}

This approach lets cron based CI flows configure a build with the latest gcc, binutils etc so they can always be tested as a workflow.

The new keyword lets a define in a nested buildset file override a command line value. That may be needed.