Merge "Always turn on TARGET_BUILD_USE_PREBUILT_SDKS for unbundled build."

<p>Unbundled build has several meanings by the context.

Let me explain the meaning by the flags related to "unbundled build"</p>

<h4>TARGET_BUILD_UNBUNDLED</h4>

<p>The source tree might not have the full platform sources. It is always set if

<code>TARGET_BUILD_APPS</code> or <code>TARGET_BUILD_UNBUNDLED_IMAGE</code> is set.</p>

<p>

    The source tree might not have the full platform sources. It turns on

    <code>TARGET_BUILD_USE_PREBUILT_SDKS</code>, unless

    <code>UNBUNDLED_BUILD_SDKS_FROM_SOURCE</code> is set. It is always set if

    <code>TARGET_BUILD_APPS</code> or <code>TARGET_BUILD_UNBUNDLED_IMAGE</code> is set.

</p>

<h4>TARGET_BUILD_USE_PREBUILT_SDKS</h4>

<p>It is an internal flag. If it is set, prebuilt SDKs are used, even if a module's

<code>LOCAL_SDK_VERSION</code> is <code>current</code> (including <code>system_current</code>,

DISABLE_PREOPT :=

ifneq (,$(TARGET_BUILD_APPS)$(TARGET_BUILD_UNBUNDLED_IMAGE))

  DISABLE_PREOPT := true

  ifndef UNBUNDLED_BUILD_SDKS_FROM_SOURCE

endif

ifeq (true,$(TARGET_BUILD_UNBUNDLED))

  ifneq (true,$(UNBUNDLED_BUILD_SDKS_FROM_SOURCE))

    TARGET_BUILD_USE_PREBUILT_SDKS := true

  endif

endif