sdk: fix APK key creation
The keys are created differently compared to the old OPKG keys. Instead
of being part of base-files/configure, they are created as a Makefile
requirement of `package/compile`, which is a cleaner solution.
This requirement would only be added to non SDK environments, however
APK always requires keys to be available. Add an `else` case for the SDK
and create keys.
Signed-off-by: Paul Spooren <[email protected]>