generate-prebuilts.sh: update paths to new key locations
generate-prebuilts.sh: update paths to new key locations

We also sign the shim here instead of hacking the upstream
Makefile to do it. MokManager has been signed with a
throwaway key per upstream design.

Change-Id: I36c910cbb5c18dc269560decdf2a06b192a9f0db
Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>
Reviewed-on: https://android.intel.com/181333
Reviewed-by: Abbas, Mohamed <cad1a51755d571b4c23450786ac5dfefb8cd1cfa@intel.com>

CyanogenMod/android_external_uefi_shim,CyanogenMod/android_external_uefi_shim,CyanogenMod/android_external_uefi_shim,android-ia/platform_external_uefi_shim,android-ia/platform_external_uefi_shim,android-ia/platform_external_uefi_shim