busybox.make: Move machine's config to separate directory

For Issue #61

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
busybox.make: Move machine's patches to separate directory

For Issue #61

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
i2ctools.make: Use cp-machine-patches to copy patches

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
busybox: have the configuration depend on optional machine configuration

Since the extra machine specific busybox config is optional it should
be added to the dependencies for $(BUSYBOX_DIR)/.config.

This will help the case during development when you update your
machine's busybox config and you want to do an incremental build.
Otherwise $(BUSYBOX_DIR)/.config will not be out of date.

Testing:

Rebuilt the Accton AS6701-32X machine which does use the extra busybox
config option.  No trouble found.

Then I touched the AS6701-32X extra busybox config, making it out of
date.  An incremental rebuild went through successfully.

Also rebuilt the fsl_p2020rdbpca machine which does *not* use the
extra busybox config option.  No trouble found.