BlackPill F401CC: add USER_BTN on PA0

Note this user buuton is not available on all revisions of this board

Fixes stm32duino#1144

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>

Author: ABOSTM

variants/STM32F4xx/F401C(B-C)_F401C(D-E)_F401CCFx/variant_BLACKPILL_F401CC.h

@@ -87,7 +87,8 @@
 
 // On-board user button
 #ifndef USER_BTN
-  #define USER_BTN              ND
+  // User button is not available on all revision of this board
+  #define USER_BTN              PA0
 #endif
 
 // SPI definitions