[target][pico] switch the uart config to target driven

Uses the target/debugconfig.h file pattern laid down before,
which is a bit long in the tooth but for the moment still helps
us separate target from platform.

target/pico/include/target/debugconfig.h

@@ -1,3 +1,14 @@
+// Copyright (c) 2020 Brian Swetland
+//
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT
+
 #pragma once
 
-#define DEBUG_UART 0
+#include <hardware/uart.h>
+
+// define how this target will map the debug uart to hardware and pins
+#define DEBUG_UART uart0
+#define DEBUG_UART_GPIOA 0
+#define DEBUG_UART_GPIOB 1

target/pico/target.c

@@ -5,9 +5,9 @@
 // https://opensource.org/licenses/MIT
 
 #include <target.h>
+#include <target/debugconfig.h>
 #include <platform/gpio.h>
 
-
 void target_early_init(void) {
 }