[][openwrt][mt7988][image][Add hkdf to mt7988.mk]

[Description]
Add hkdf to mt7988.mk to generate kdf key for firmware encryption.

[Release-log]
N/A

Change-Id: I86dde63b632fa26f235d8a5b92fd5ad4577a28da
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8106486
1 file changed