Make tickless kernel the default mode

[nashif]

Move to tickless kernel by default.

Should address the issues below as well as set the default to tickless kernel

• add tickless idle and kernel support to RISCV32 pulpino #3651: add tickless idle and kernel support to RISCV32 pulpino
• Tickless Kernel Timekeeping Problem #7048: Tickless Kernel Timekeeping Problem
• tickless and timeslicing don't play well together #7193: tickless and timeslicing don't play well together
• Unify TICKLESS_IDLE & TICKLESS_KERNEL #8257: Unify KERNEL_TICKLESS and KERNEL_IDLE
• Clean up timeout handling #9627: Clean up timeout handling
• System timer handling with low-frequency timers #9904: nRF system timer handling

[carlescufi]

CC @pizi-nordic @cvinayak

[andyross]

Refactoring in preparation up at #10160 -- that shrinks the driver interface (a lot) and cleans up the intent of the behavior. It doesn't actually change the way the kernel or drivers work, but it enables a simpler rewrite of both.

[galak]

@andyross @nashif is this resolved, can we close this?

[andyross]

Oh, yeah. Long since merged, sorry.