sandbox: improve description of CONFIG_SANDBOX_CRASH_RESET

Mentions that command line option --signal is needed to make use of this
configuration option.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/arch/sandbox/Kconfig b/arch/sandbox/Kconfig
index 0ce77de..1c8353d 100644
--- a/arch/sandbox/Kconfig
+++ b/arch/sandbox/Kconfig
@@ -58,10 +58,15 @@
 	bool "Reset on crash"
 	help
 	  If an illegal instruction or an illegal memory access occurs, the
-	  sandbox by default writes a crash dump and exits. If you set this
-	  flag, the sandbox is reset instead. This may be useful when running
-	  test suites like the UEFI self certification test which continue
-	  with the next test after a crash.
+	  sandbox exits with an error by default.
+
+	  If the u-boot binary is invoked with --signals (or -S), U-Boot will
+	  handle the signal writing a crash dump before exiting.
+
+	  If you additionally set the CONFIG_SANDBOX_CRASH_RESET flag, the
+	  sandbox is reset after writing the crash dump. This may be useful
+	  when running test suites like the UEFI self certification test which
+	  continue with the next test after a crash.
 
 config SANDBOX_BITS_PER_LONG
 	int