cmd: sandbox: implement exception command

Implement the commands

* exception undefined - execute an illegal instruction
* exception sigsegv - cause a segment violation

Here is a possible output:

    => exception undefined
    Illegal instruction
    pc = 0x55eb8d0a7575, pc_reloc = 0x57575
    Resetting ...

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Makefile b/cmd/Makefile
index dd86675..5b3400a 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -193,6 +193,7 @@
 
 obj-$(CONFIG_ARM) += arm/
 obj-$(CONFIG_RISCV) += riscv/
+obj-$(CONFIG_SANDBOX) += sandbox/
 obj-$(CONFIG_X86) += x86/
 
 obj-$(CONFIG_ARCH_MVEBU) += mvebu/