Builds fail when using devtool modify

[nslowell]

If you use devtool modify to check out or point to outside source code, then

poky/scripts/lib/devtool/standard.py executes the following code to add to do_configure():

            if rd.getVarFlag('do_menuconfig','task'):
                f.write('\ndo_configure:append() {\n'
                '    if [ ! ${DEVTOOL_DISABLE_MENUCONFIG} ]; then\n'
                '        cp ${B}/.config ${S}/.config.baseline\n'
                '        ln -sfT ${B}/.config ${S}/.config.new\n'
                '    fi\n'
                '}\n')

and since DEVTOOL_DISABLE_MENUCONFIG is not set anywhere and the .config file does not seem to exist,
when you attempt to build, do_configure will fail with the error:
cp: cannot stat '${B}/.config': No such file or directory

Reproduce:

1. follow README setup instructions for qemu zephyr-philosophers
2. devtool modify zephyr-philosophers ~/zephyr-philosophers-src
3. bitbake zephyr-philosophers
4. observe failure