sandbox: unblock signal before calling execv()

sandbox: unblock signal before calling execv()

The following faulty behavior was observed. The sandbox configured with
CONFIG_SANDBOX_CRASH_RESET=y was invoked with

    ./u-boot -T -S

After executing `exception undefined' the sandbox reboots.
When executing `exception undefined' the sandbox exits with SIGSEGV.

The expected behavior is that the sandbox should reboot again.

If we are relaunching the sandbox in a signal handler, we have to unblock
the respective signal before calling execv(). See signal(7) man-page.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>