ipc-lld-fw: fix ALTERNATIVE use as package-specific
ipc-lld-fw: fix ALTERNATIVE use as package-specific

Previous commit, while adding support for another platform, extended
ALTERNATIVE variable (which is package-specific) with machine overrides,
accidentally dropping the necessary ${PN} suffix in the process.
Restore it to make update-alternatives system work again.

Signed-off-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>

rcn-ee/meta-ti,rcn-ee/meta-ti,rcn-ee/meta-ti,rcn-ee/meta-ti