bootstrap: Ratchet up compiler strictness

Replace the specific warning flags with -Wall -Wextra and turn on
-Werror.

Covered by -Wall:

* -Wformat
* -Warray-bounds
* -Wimplicit-function-declaration

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Iab21e67026d2453648aa6ebe4c299e1162fc5ebf
diff --git a/tests/test_serial.c b/tests/test_serial.c
index eef66d0..12d1476 100644
--- a/tests/test_serial.c
+++ b/tests/test_serial.c
@@ -33,7 +33,8 @@
 	ssize_t rc;
 
 	rc = write(ctx->egress, buf, len);
-	assert(rc == len);
+	assert(rc >= 0);
+	assert((size_t)rc == len);
 
 	return rc;
 }
@@ -42,7 +43,10 @@
 
 static bool seen;
 
-static void rx_message(uint8_t eid, void *data, void *msg, size_t len)
+#define __unused __attribute__((unused))
+
+static void rx_message(uint8_t eid __unused, void *data __unused, void *msg,
+		       size_t len)
 {
 	uint8_t type;