Save memory (move usb mass storage buffer to reusableBuffer) (#5579)

* Save 128 byte by moving variable to stack

* Make usb mass storage buffer smaller on F2 processors and put into reuseablebuffer

- Lower USB mass storage buffer to 512 bytes per default on F2
  targets, makes usb speed slightly slower ~600 kB/s instead ~700 KB/s
  in my tests, saving 3584 bytes
- Save another ~300 bytes by putting that buffer into reusableBuffer

* Don't put id into reusuableBuffer

All uses of id can be on the stack as well. Makes no difference for ram usage but makes it safer to reuse reusableBuffer

* Only link cdc serial when needed

We always linked usb cdc even in non debug builds. CDC uses about ~600 byte of RAM for its buffers and structures.

radio/src/mixer.cpp

@@ -1089,7 +1089,6 @@ void evalFlightModeMixes(uint8_t mode, uint8_t tick10ms)
   mixWarning = lv_mixWarning;
 }
 
-int32_t sum_chans512[MAX_OUTPUT_CHANNELS] = {0};
 
 
 #define MAX_ACT 0xffff
@@ -1102,6 +1101,7 @@ uint8_t   flightModeTransitionLast = 255;
 
 void evalMixes(uint8_t tick10ms)
 {
+  int32_t sum_chans512[MAX_OUTPUT_CHANNELS] = {0};
 #if defined(PCBMEGA2560) && defined(DEBUG) && !defined(VOICE)
   PORTH |= 0x40; // PORTH:6 LOW->HIGH signals start of mixer interrupt
 #endif