phase2: fix signing steps when only apk_key is defined

phase2: fix signing steps when only apk_key is defined

Signing steps are currently skipped if only APK signing is configured,
because phase2 effectively enables signing only when `usign` is present.

Fix this by making `IsSignEnabled` explicitly cover APK signing too.

While at it, refactor the signing checks into dedicated helper functions
`IsUsignEnabled`, `IsApkSigningEnabled`, and `IsGpgSigningEnabled`, and
use them consistently to align phase2 with the phase1 implementation.

Signed-off-by: Petr Štetiar <[email protected]>