build: generate private key for APK early

build: generate private key for APK early

Other than OPKG which only uses signed package list, APK uses
individually signed packages in addition to signed package lists. Hence,
in order to be able to generate package, the private key needs to be
generated before compiling packages. Express that dependency and
generate the private key before building any packages instead of doing
so as part of the base-files package build.

Fixes: d788ab376f ("build: add APK package build capabilities")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>