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/bf548-ezkit_defconfig b/configs/bf548-ezkit_defconfig
index 6d398ac..e4fa136 100644
--- a/configs/bf548-ezkit_defconfig
+++ b/configs/bf548-ezkit_defconfig
@@ -16,5 +16,6 @@
 CONFIG_CMD_FAT=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_USB=y
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y
 CONFIG_LIB_RAND=y