Some components (eg. GMP) will fail to correctly build if
the CONFIG_SHELL is not bash (eg. ash or dash). So make bash
the default CONFIG_SHELL.
Keep ash as a possible selection, as future versions of those
currently /broken/ tools may come fixed wrt to CONFIG_SHELL
being POSIX-ly compliant.
It apears that more and more ./configure scripts and Makefiles
make use of non-POSIXly correct shell constructs, that don't
work with ash.
For now, just warn the user against using ash, but keep it as
an option in case newer versions of the /broken/ components
come fixed, and we can again use ash, as it is in some cases
really faster than bash.
In case the shell the user wants to use as CONFIG_SHELL is located in a
weird place (eg. /opt/bash/bin/bash), or is weirdly named (eg. bash-4),
let the user enter the patch to the shell.
