Only propagate register errors on some targets

Without this change, connecting to ARM targets is impossible.

Fixes #115.

Change-Id: Ie33c7e15ac1bed8c9cbd8e6a78de92d5498c5999

src/target/riscv/riscv.c

@@ -245,6 +245,7 @@ static int riscv_init_target(struct command_context *cmd_ctx,
 		struct target *target)
 {
 	LOG_DEBUG("riscv_init_target()");
+	target->propagate_register_errors = true;
 	target->arch_info = calloc(1, sizeof(riscv_info_t));
 	if (!target->arch_info)
 		return ERROR_FAIL;

src/target/target.h

@@ -204,6 +204,14 @@ struct target {
 
 	/* file-I/O information for host to do syscall */
 	struct gdb_fileio_info *fileio_info;
+
+	/**
+	 * When true, send gdb an error result when reading/writing a register
+	 * fails. This must be false for some ARM targets (Cortex-M3), where a 'g'
+	 * packet results in an attempt to read 'r0', which fails, which causes gdb
+	 * to close the connection.
+	 */
+	bool propagate_register_errors;
 };
 
 struct target_list {