cmd: add exception command

The 'exception' command allows to test exception handling.

This implementation supports ARM, x86, RISC-V and the following exceptions:
* 'breakpoint' - prefetch abort exception (ARM 32bit only)
* 'unaligned'  - data abort exception (ARM only)
* 'undefined'  - undefined instruction exception

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

include/exception.h

@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * The 'exception' command can be used for testing exception handling.
+ *
+ * Copyright (c) 2018, Heinrich Schuchardt <xypron.glpk@gmx.de>
+ */
+
+static int do_exception(cmd_tbl_t *cmdtp, int flag, int argc,
+			char * const argv[])
+{
+	cmd_tbl_t *cp;
+
+	if (argc != 2)
+		return CMD_RET_USAGE;
+
+	/* drop sub-command parameter */
+	argc--;
+	argv++;
+
+	cp = find_cmd_tbl(argv[0], cmd_sub, ARRAY_SIZE(cmd_sub));
+
+	if (cp)
+		return cp->cmd(cmdtp, flag, argc, argv);
+
+	return CMD_RET_USAGE;
+}
+
+static int exception_complete(int argc, char * const argv[], char last_char,
+			      int maxv, char *cmdv[])
+{
+	int len = 0;
+	int i = 0;
+	cmd_tbl_t *cmdtp;
+
+	switch (argc) {
+	case 1:
+		break;
+	case 2:
+		len = strlen(argv[1]);
+		break;
+	default:
+		return 0;
+	}
+	for (cmdtp = cmd_sub; cmdtp != cmd_sub + ARRAY_SIZE(cmd_sub); cmdtp++) {
+		if (i >= maxv - 1)
+			return i;
+		if (!strncmp(argv[1], cmdtp->name, len))
+			cmdv[i++] = cmdtp->name;
+	}
+	cmdv[i] = NULL;
+	return i;
+}
+
+U_BOOT_CMD_COMPLETE(
+	exception, 2, 0, do_exception,
+	"Forces an exception to occur",
+	exception_help_text, exception_complete
+);