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>

config/Config-kernel.in

@@ -806,3 +806,25 @@ config KERNEL_DEVKMEM
 	  Say Y here if you want to support the /dev/kmem device. The
 	  /dev/kmem device is rarely used, but can be used for certain
 	  kind of kernel debugging operations.
+
+#
+# compile optimiziation setting
+#
+choice
+	prompt "Compiler optimization level"
+	default KERNEL_CC_OPTIMIZE_FOR_SIZE if SMALL_FLASH
+
+config KERNEL_CC_OPTIMIZE_FOR_PERFORMANCE
+	bool "Optimize for performance"
+	help
+	  This is the default optimization level for the kernel, building
+	  with the "-O2" compiler flag for best performance and most
+	  helpful compile-time warnings.
+
+config KERNEL_CC_OPTIMIZE_FOR_SIZE
+	bool "Optimize for size"
+	help
+	  Enabling this option will pass "-Os" instead of "-O2" to
+	  your compiler resulting in a smaller kernel.
+
+endchoice