[config] Allow user to specify custom CONFIG_SHELL
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.
1 This is NOT the official version of dialog. This version has been
2 significantly modified from the original. It is for use by the Linux
3 kernel configuration script. Please do not bother Savio Lam with