kbuild, blackfin: Add CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED

Many (but not all) of Blackfin boards give -O2 option
to compile under lib/ directory.
That means lib/ should be speed-optimized,
whereas other parts should be size-optimized.

We want to keep the same behavior,
but do not want to parse board/*/config.mk again and again.
We've got no choice but to invent a new method.

CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED, if it is enabled,
gives -O2 flag only for building under lib/ directory.

Dirty codes which I had marked as "FIX ME"
in board/${BOARD}/config.mk have been deleted.
Instead, CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED has been
defined in include/configs/${BOARD}.h.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/include/configs/bf561-ezkit.h b/include/configs/bf561-ezkit.h
index fb6f948..0a309d9 100644
--- a/include/configs/bf561-ezkit.h
+++ b/include/configs/bf561-ezkit.h
@@ -102,6 +102,7 @@
  * Misc Settings
  */
 #define CONFIG_UART_CONSOLE	0
+#define CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED
 
 /*
  * Run core 1 from L1 SRAM start address when init uboot on core 0