kernel: Fix the check for official builds

The check for official builds now only affects the case where a prebuilt
kernel was actually used, not the case where a device builds a kernel
from source as normal.

The check has also been turned around so that it only fails the build if
it's actually an official build, not an unofficial one.

Change-Id: I1280b16b90589742c2abe7f3f6f5975a3aaeea03

build/tasks/kernel.mk

@@ -156,10 +156,10 @@ else
         $(warning **********************************************************)
         $(error "NO KERNEL CONFIG")
     else
-        ifeq ($(filter RELEASE NIGHTLY SNAPSHOT EXPERIMENTAL,$(LINEAGE_BUILDTYPE)),)
-            $(error "PREBUILT KERNEL IS NOT ALLOWED ON OFFICIAL BUILDS!")
-        else
-            ifneq ($(TARGET_FORCE_PREBUILT_KERNEL),)
+        ifneq ($(TARGET_FORCE_PREBUILT_KERNEL),)
+            ifneq ($(filter RELEASE NIGHTLY SNAPSHOT EXPERIMENTAL,$(LINEAGE_BUILDTYPE)),)
+                $(error "PREBUILT KERNEL IS NOT ALLOWED ON OFFICIAL BUILDS!")
+            else
                 $(warning **********************************************************)
                 $(warning * Kernel source found and configuration was defined,     *)
                 $(warning * but prebuilt kernel is being forced.                   *)
@@ -172,10 +172,10 @@ else
                 $(warning **********************************************************)
                 FULL_KERNEL_BUILD := false
                 KERNEL_BIN := $(TARGET_PREBUILT_KERNEL)
-            else
-                FULL_KERNEL_BUILD := true
-                KERNEL_BIN := $(TARGET_PREBUILT_INT_KERNEL)
             endif
+        else
+            FULL_KERNEL_BUILD := true
+            KERNEL_BIN := $(TARGET_PREBUILT_INT_KERNEL)
         endif
     endif
 endif