Even we build the board ROM based on AOSP, there is no Google Play Store built-in. Further more, the Google Apps (GAPPS) are closed-source.

How to install Google Play Store, or build into the ROM? How to deal with the License issue?

You can download and rebuilt the system image involving the following APKs form APKMirror with suitable versions to enable the Google Play Store on your board:

• Google Play Store
• Google Play Services
• Google Services Framework
• Google Account Manager

So far, we have no License or Sub-License of Google Play Store that can be combined with X20 96Boards.
I suggest you contact Google to figure out the license issue.

Meanwhile, you can consider a alternative method to satisfy your needs, microG:

microG GmsCore is a free and open implementation of the Google Play Services Framework. It allows applications calling proprietary Google APIs to run on AOSP-based ROMs like Replicant and CyanogenMod. Acting as a replacement for the closed-source Google Apps (GAPPS), it is a powerful tool to reclaim your privacy while enjoying Android core features.

To figure out the right version of Google Play services for your Android device, download Play Services info or go to Settings -> Apps -> Google Play services, and look at the 3-digit sequence (let's use XYZ) in the parentheses:

* X defines Android version:

• 0 for Android <5.0
• 2 for Android 5.0 and 5.1 starting with Play Services v8
• 4 for Android 5.0 and 5.1 before Play Services v8, Android >= 6.0 after
• 5 for Android Wear
• 7 for Android 5.0
• 8 for Android TV

* Y defines CPU architecture:

• 1 for armeabi
• 3 for armeabi-v7a
• 4 for arm64-v8a
• 5 for mips
• 7 for x86
• 8 for x86_64

* Z defines DPI:

• 0 for universal
• 2 for 160
• 4 for 240
• 6 for 320
• 8 for 480

Notes:

• It seems that 7YZ builds became 4YZ with Google Play services v6.5. If you were on 7YZ before, you should now install 4YZ.
• It seems that 4YZ builds got split into 2YZ for Lollipop and 4YZ for Marshmallow with Google Play services v8. If you were on 4YZ before, you should now install 2YZ in Lollipop and 4YZ in Marshmallow.

Examples:

• 014 for Android <5.0 armeabi CPU 240 DPI device
• 438 for Android 6.0+ armeabi-v7a CPU 480 DPI device
• 876 for Android TV x86 CPU 320 DPI device