AP_HAL_Linux: 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_Linux/UARTDriver.cpp

@@ -281,18 +281,17 @@ uint32_t UARTDriver::txspace()
     return _writebuf.space();
 }
 
-int16_t UARTDriver::read()
+bool UARTDriver::read(uint8_t &byte)
 {
     if (!_initialised) {
-        return -1;
+        return false;
     }
 
-    uint8_t byte;
     if (!_readbuf.read_byte(&byte)) {
-        return -1;
+        return false;
     }
 
-    return byte;
+    return true;
 }
 
 bool UARTDriver::discard_input()

libraries/AP_HAL_Linux/UARTDriver.h

@@ -29,7 +29,7 @@ public:
     /* Linux implementations of Stream virtual methods */
     uint32_t available() override;
     uint32_t txspace() override;
-    int16_t read() override;
+    bool read(uint8_t &b) override WARN_IF_UNUSED;
 
     bool discard_input() override;