Merge "Document unbundle build behavior"

LOCAL_GENERATED_SOURCES += $(GEN)

</pre>

<h3><a name="unbundled-build"/>Unbundled build</h3>

<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>

<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>,

<code>core_current</code>, and so on). If it is unset, build current SDKs,

and use them as usual.</p>

<h4>DISABLE_PREOPT</h4>

<p>It is an internal flag as well. If it is set, dexpreopt is disabled.

It is always set if <code>TARGET_BUILD_APPS</code> or <code>TARGET_BUILD_UNBUNDLED_IMAGE</code> is set,

because dexpreopt tightly depends on the platform.</p>

<h4>TARGET_BUILD_APPS</h4>

<p>Build the apps that can be distributed outside the platform, so it turns on

<code>TARGET_BUILD_UNBUNDLED</code> and <code>DISABLE_PREOPT</code>.

Also, it turns on <code>TARGET_BUILD_USE_PREBUILT_SDKS</code>, unless

<code>UNBUNDLED_BUILD_SDKS_FROM_SOURCE</code> is set.</p>

<h4>TARGET_BUILD_UNBUNDLED_IMAGE</h4>

<p>It is similar to <code>TARGET_BUILD_APPS</code>, but its target is an unbundled partition

(such as the vendor partition). Accordingly, it sets <code>TARGET_BUILD_UNBUNDLED</code> and <code>DISABLE_PREOPT</code>.

We can call the partition unbundled, because the partition can be distributed outside the platform.

And also, it turns on <code>TARGET_BUILD_USE_PREBUILT_SDKS</code>, unless

<code>UNBUNDLED_BUILD_SDKS_FROM_SOURCE</code> is set.</p>

<h3><a name="platform-specific"/>Platform specific conditionals</h3>

<p>Sometimes you need to set flags specifically for different platforms.  Here

is a list of which values the different build-system defined variables will be