configs: blackfin: move CONFIG_USB to defconfig

These Blackfin boards are the last ones that define CONFIG_USB in
their headers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/configs/bf527-ezkit-v2_defconfig b/configs/bf527-ezkit-v2_defconfig
index ff797f8..faac28d 100644
--- a/configs/bf527-ezkit-v2_defconfig
+++ b/configs/bf527-ezkit-v2_defconfig
@@ -17,4 +17,5 @@
 CONFIG_CMD_FAT=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_USB=y
 CONFIG_LIB_RAND=y