commands: add intentionally UB triggering ubsan command ...

For testing whether ubsan works, triggering undefined behavior to detect
is a nice development aid. Port the Linux test_ubsan module to barebox
as a command.

barebox@Embest MarS Board i.MX6Dual:/ ubsan shift
ERROR: ================================================================================
ERROR: UBSAN: Undefined behaviour in commands/ubsan.c:53:7
ERROR: shift exponent -1 is negative
WARNING: [<4fd77325>] (unwind_backtrace+0x1/0x68) from [<4fd5bc0f>] (ubsan_epilogue.isra.6+0x7/0x20)
WARNING: [<4fd5bc0f>] (ubsan_epilogue.isra.6+0x7/0x20) from [<4fd5bf8d>] (__ubsan_handle_shift_out_of_bounds+0x49/0xb8)
WARNING: [<4fd5bf8d>] (__ubsan_handle_shift_out_of_bounds+0x49/0xb8) from [<4fd51ed7>] (test_ubsan_shift_out_of_bounds+0x23/0x2c)
WARNING: [<4fd51ed7>] (test_ubsan_shift_out_of_bounds+0x23/0x2c) from [<4fd51fd7>] (do_ubsan+0x3b/0x54)
WARNING: [<4fd51fd7>] (do_ubsan+0x3b/0x54) from [<4fd03a4d>] (execute_command+0x21/0x48)
WARNING: [<4fd03a4d>] (execute_command+0x21/0x48) from [<4fd09591>] (run_list_real+0x5b5/0x610)
WARNING: [<4fd09591>] (run_list_real+0x5b5/0x610) from [<4fd08ed9>] (parse_stream_outer+0x105/0x164)
WARNING: [<4fd08ed9>] (parse_stream_outer+0x105/0x164) from [<4fd097b1>] (run_shell+0x35/0x64)
WARNING: [<4fd097b1>] (run_shell+0x35/0x64) from [<4fd00d43>] (run_init+0x8f/0x168)
WARNING: [<4fd00d43>] (run_init+0x8f/0x168) from [<4fd00e35>] (start_barebox+0x19/0x54)
WARNING: [<4fd00e35>] (start_barebox+0x19/0x54) from [<4fd75843>] (barebox_non_pbl_start+0xc7/0x108)
WARNING: [<4fd75843>] (barebox_non_pbl_start+0xc7/0x108) from [<4fd00005>] (__bare_init_start+0x1/0xc)
ERROR: ================================================================================

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>