target/linux/config: allow for building kernel with -Os

As this is potentially preferred on an embedded system, make it the
default.

Signed-off-by: Phil Sutter <phil@nwl.cc>

target/linux/config/Config.in.kernel

@@ -189,4 +189,9 @@ config ADK_KERNEL_PREEMPT
 	boolean
 	default n
 
+config ADK_KERNEL_CC_OPTIMIZE_FOR_SIZE
+	prompt "Optimize for size"
+	boolean
+	default y
+
 endmenu