unbreak bin/menuconfig on kernel sources > 5.12

[telent]

In linux commit f91e46b1a722 (~ 5.12) the makefile rules for {menu,n,g,x,}config were unified using a make definition, which involved doubling the dollar sign from $< to $$<.

This broke a piece of sed magic in the kernel builder.

[samueldr]

At a glance, the change looks good; I'll have to test it against an older and a newer kernel to make sure things are working, though.

[samueldr]

Thank you!