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_defconfig b/configs/bf527-ezkit_defconfig
index eff2a12..d69b146 100644
--- a/configs/bf527-ezkit_defconfig
+++ b/configs/bf527-ezkit_defconfig
@@ -17,3 +17,4 @@
 # CONFIG_NET_TFTP_VARS is not set
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_USB=y