AP_HAL: add and use a "bool read(c)" method to AP_HAL

this is much less likely to not work vs the int16_t equivalent

libraries/AP_HAL/UARTDriver.h

@@ -66,7 +66,7 @@ public:
     virtual size_t write_locked(const uint8_t *buffer, size_t size, uint32_t key) { return 0; }
 
     // read from a locked port. If port is locked and key is not correct then 0 is returned
-    virtual int16_t read_locked(uint32_t key) { return -1; }
+    virtual bool read_locked(uint32_t key, uint8_t &b) WARN_IF_UNUSED { return -1; }
     
     // control optional features
     virtual bool set_options(uint16_t options) { _last_options = options; return options==0; }

libraries/AP_HAL/Util.cpp

@@ -36,7 +36,7 @@ public:
     const size_t _size;
 
     uint32_t available() override { return 0; }
-    int16_t read() override { return -1; }
+    bool read(uint8_t &b) override { return false; }
     uint32_t txspace() override { return 0; }
     bool discard_input() override { return false; }
 };

libraries/AP_HAL/utility/BetterStream.cpp

@@ -20,6 +20,15 @@ size_t AP_HAL::BetterStream::write(const char *str)
     return write((const uint8_t *)str, strlen(str));
 }
 
+int16_t AP_HAL::BetterStream::read()
+{
+    uint8_t b;
+    if (!read(b)) {
+        return -1;
+    }
+    return b;
+}
+
 ssize_t AP_HAL::BetterStream::read(uint8_t *buffer, uint16_t count) {
     uint16_t offset = 0;
     while (count--) {

libraries/AP_HAL/utility/BetterStream.h

@@ -40,9 +40,8 @@ public:
 
     virtual uint32_t available() = 0;
 
-    /* return value for read():
-     * -1 if nothing available, uint8_t value otherwise. */
-    virtual int16_t read() = 0;
+    int16_t read();  // old function; prefer calling the boolean method
+    virtual bool read(uint8_t &b) WARN_IF_UNUSED = 0;
 
     // no base-class implementation to force descendants to
     // do things efficiently.  Looping over 2^32-1 bytes would be bad.