meta-aspeed: u-boot-aspeed-sdk: Ensure SOCSEC_SIGN defaults are set

Include the socsec-sign class after the conditional assignments to
ensure the values we might assign are considered by the functions of
socsec-sign.

Without this, u-boot-aspeed-sdk do_deploy logs show:

```
DEBUG: Executing python function sstate_task_prefunc
DEBUG: Python function sstate_task_prefunc finished
DEBUG: Executing shell function do_deploy
Copying u-boot-nodtb binary...
Warning: Invalid socsec signing key - SPL verified boot won't be available
```

Adjusting the location of the include gives:

```
DEBUG: Executing python function sstate_task_prefunc
DEBUG: Python function sstate_task_prefunc finished
DEBUG: Executing shell function do_deploy
Copying u-boot-nodtb binary...
check header PASS
Found PEM header at position 381
check integrity PASS
```

Change-Id: I15cac4c315a6ceaeb69b0c02ba0b7d05e54f6e15
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>