Build scripts for android arm/x86/mips

Correct buildscripts for building library for android in each main archs: arm/x86/mips.

Some comments:
LIBSODIUM_LD_FLAGS introduced because android have no support for versioned libraries (eg libsodium.so.0), so I make this env variable and pass flag "-avoid-version" to linker.

NDK_ROOT -> ANDROID_NDK_HOME: this name convention uses Google in some build tools, eg android-gradle-plugin.