kernel: only optimized for size if small_flash

Add a new config option to allow to select the default compile
optimization level for the kernel.

Select the optimization for size by default if the small_flash feature is
set. Otherwise "Optimize for performance" is set.

Add the small_flash feature flag to all (sub)targets which had the
optimization for size in their default kernel config.

Remove CC_OPTIMIZE_FOR_* symbols from all kernel configs to apply the new
setting.

Exceptions to the above are:

  - lantiq, where the optimization for size is only required for the
    xway_legacy subtarget but was set for the whole target
  - mediatek, ramips/mt7620 & ramips/mt76x8 where boards should have
    plenty of space and an optimization for size doesn't make much sense
  - rb532, which has 128MByte flash

Signed-off-by: Mathias Kresin <dev@kresin.me>