Kbuild: change CC_OPTIMIZE_FOR_SIZE definition

CC_OPTIMIZE_FOR_SIZE disables the often useful -Wmaybe-unused warning,
because that causes a ridiculous amount of false positives when combined
with -Os.

This means a lot of warnings don't show up in testing by the developers
that should see them with an 'allmodconfig' kernel that has
CC_OPTIMIZE_FOR_SIZE enabled, but only later in randconfig builds
that don't.

This changes the Kconfig logic around CC_OPTIMIZE_FOR_SIZE to make
it a 'choice' statement defaulting to CC_OPTIMIZE_FOR_PERFORMANCE
that gets added for this purpose. The allmodconfig and allyesconfig
kernels now default to -O2 with the maybe-unused warning enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Marek <mmarek@suse.com>

init/Kconfig

@@ -1283,6 +1283,17 @@ source "usr/Kconfig"
 
 endif
 
+choice
+	prompt "Compiler optimization level"
+	default CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
+
+config 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 CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size"
 	help
@@ -1291,6 +1302,8 @@ config CC_OPTIMIZE_FOR_SIZE
 
 	  If unsure, say N.
 
+endchoice
+
 config SYSCTL
 	bool