Load the kernel config file where userconfig (with option USERCONFIG_BOOT)

Load the kernel config file where userconfig (with option USERCONFIG_BOOT)
will see it.  The name of the file is obtained by appending ".config"
to the name of the kernel file.  If the file doesn't exist, then an
annoying message is printed.

Reduced the command line buffer size yet again from 4096 to 1024.  It is
oversized to defend against overrun in gets(), but data space is starting
to run out so the buffer size can't be as grossly oversized as before.