boot_serial: support fragmentation for outgoing SMP packets

The mcumgr packet sent over serial should be fragmented into frames of
up to 127 bytes, including 2-bytes frame start header and 1-byte for
terminating newline [1], resulting in up to 124 bytes for the base64
encoded payload.

Current implementation doesn't fulfill above requirement and transmits
single frames, without considering their length. This change introduces
support for fragmenting as defined in 'SMP over console' specification.

[1] github.com/apache/mynewt-mcumgr/blob/master/transport/smp-console.md

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

Author: pepe2k

boot/boot_serial/src/boot_serial.c

@@ -83,6 +83,7 @@ BOOT_LOG_MODULE_DECLARE(mcuboot);
 #endif
 
 #define BOOT_SERIAL_OUT_MAX     (128 * BOOT_IMAGE_NUMBER)
+#define BOOT_SERIAL_FRAME_MTU   124 /* 127 - pkt start (2 bytes) and stop (1 byte) */
 
 #ifdef __ZEPHYR__
 /* base64 lib encodes data to null-terminated string */
@@ -758,9 +759,10 @@ static void
 boot_serial_output(void)
 {
     char *data;
-    int len;
+    int len, out;
     uint16_t crc;
     uint16_t totlen;
+    char pkt_cont[2] = { SHELL_NLIP_DATA_START1, SHELL_NLIP_DATA_START2 };
     char pkt_start[2] = { SHELL_NLIP_PKT_START1, SHELL_NLIP_PKT_START2 };
     char buf[BOOT_SERIAL_OUT_MAX + sizeof(*bs_hdr) + sizeof(crc) + sizeof(totlen)];
     char encoded_buf[BASE64_ENCODE_SIZE(sizeof(buf))];
@@ -786,8 +788,6 @@ boot_serial_output(void)
 #endif
     crc = htons(crc);
 
-    boot_uf->write(pkt_start, sizeof(pkt_start));
-
     totlen = len + sizeof(*bs_hdr) + sizeof(crc);
     totlen = htons(totlen);
 
@@ -810,8 +810,23 @@ boot_serial_output(void)
 #else
     totlen = base64_encode(buf, totlen, encoded_buf, 1);
 #endif
-    boot_uf->write(encoded_buf, totlen);
-    boot_uf->write("\n", 1);
+
+    out = 0;
+    while (out < totlen) {
+        if (out == 0) {
+            boot_uf->write(pkt_start, sizeof(pkt_start));
+        } else {
+            boot_uf->write(pkt_cont, sizeof(pkt_cont));
+        }
+
+        len = MIN(BOOT_SERIAL_FRAME_MTU, totlen - out);
+        boot_uf->write(&encoded_buf[out], len);
+
+        out += len;
+
+        boot_uf->write("\n", 1);
+    }
+
     BOOT_LOG_INF("TX");
 }