nanohub: bl: fix compile error for cvprintf()

Commit 1e9ebe0cdde2520bf0568ad1da2a65b8ac9f4d93 ("nanohub:
remove -fshort-double") changes the prototype for cvprintf()
triggering a compile error when DEBUG_UART_PIN is defined.

Fix it by aligning call to cvprintf() to the new prototype.

Change-Id: I033cd765bc03174dbc624462aceaa81183831e52
Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
diff --git a/firmware/os/core/bl.c b/firmware/os/core/bl.c
index 706a749..23d7f5b 100644
--- a/firmware/os/core/bl.c
+++ b/firmware/os/core/bl.c
@@ -110,7 +110,7 @@
     va_list vl;
 
     va_start(vl, str);
-    cvprintf(blLogPutcharF, NULL, str, vl);
+    cvprintf(blLogPutcharF, 0, NULL, str, vl);
     va_end(vl);
 }