busybox: flashing from USB needs `blkdiscard`

...and apparently it is not getting built since the latest Buildroot
update.

Change-Id: I1b781b3a55b9e2952f4c4874508ed68d175ec788
diff --git a/board/czechlight/common/busybox-config b/board/czechlight/common/busybox-config
index af0efe0..174c853 100644
--- a/board/czechlight/common/busybox-config
+++ b/board/czechlight/common/busybox-config
@@ -5,3 +5,6 @@
 # rauc
 CONFIG_FEATURE_TAR_AUTODETECT=y
 CONFIG_FEATURE_SEAMLESS_XZ=y
+
+# initial flashing
+CONFIG_BLKDISCARD=y