scripts/feeds: Prevent .config autocreation

When using scripts/feeds upgrade the .config needs to be
updated but the code to do so was also autocreating a
.config if one didn't exist.  This is counter-productive
when you have not yet used menuconfig (or other config targets)
because things like selecting or deselecting CONFIG_ALL
(to build all package by default) only works if the
package selection has not already been done via an
existing .config selection.

Signed-off-by: Daniel Dickinson <lede@daniel.thecshore.com>