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/bf526-ezbrd_defconfig b/configs/bf526-ezbrd_defconfig
index 85c2a6b..ea4f8e4 100644
--- a/configs/bf526-ezbrd_defconfig
+++ b/configs/bf526-ezbrd_defconfig
@@ -17,3 +17,4 @@
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SST=y
+CONFIG_USB=y