ndk-build: let the user do the switch for setting the prefix
ndk-build: let the user do the switch for setting the prefix

It's recommended that this switch is done in Android.mk. It let
us use keep the old behaviour using GSTREAMER_SDK_ROOT and users
might want to set different roots based on other conditionals
such as the debug/nodebug one.

BigBrother-International/gst-cerbero,BigBrother-International/gst-cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,BigBrother-International/gst-cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,BigBrother-International/gst-cerbero,BigBrother-International/gst-cerbero