I'm currently testing a custom board using BSP_SIMPLE defined in my board header file.
This is failing to build due to undefined BSP_LED_APP_TIMERS_NUMBER.
Appears to be due to a simple error at line 61 in bsp.h, which reads...
#define BSP_APP_APP_TIMERS_NUMBER 0
...instead of...
#define BSP_LED_APP_TIMERS_NUMBER 0
A simple workaround for me is to add this to the custom board header file instead.