Enable ftrace in default kernel configs

[ddiss]

ftrace is a useful debugging utility, and can be configured with little impact on performance, with extra per-function NOPs added and used as tracing trampolines.

Building the SLE15SP2 kernel (I'd expect similar for mainline) with ftrace enabled results in the following size increase:
before: 4239920 ./arch/x86/boot/bzImage
after: 4870704 ./arch/x86/boot/bzImage

[morbidrsa]

Sounds like a good idea to me, at least on vanilla. I'm always having tracepoints enabled in my config