switch to standard NDK env var: ANDROID_NDK_HOME

ANDROID_NDK_HOME is hardcoded in gradle as the name of the env var:
https://android.googlesource.com/platform/tools/build/+/master/gradle/src/main/groovy/com/android/build/gradle/internal/Sdk.groovy

external/Makefile

@@ -32,10 +32,10 @@ else
 endif
 
 # Android NDK setup
-NDK_BASE ?= /opt/android-ndk
+ANDROID_NDK_HOME ?= /opt/android-ndk
 NDK_ABI=arm
 # NDK platform level, aka APP_PLATFORM, is equivalent to minSdkVersion
-NDK_SYSROOT=$(NDK_BASE)/platforms/$(APP_PLATFORM)/arch-$(NDK_ABI)
+NDK_SYSROOT=$(ANDROID_NDK_HOME)/platforms/$(APP_PLATFORM)/arch-$(NDK_ABI)
 NDK_UNAME := $(shell uname -s | tr '[A-Z]' '[a-z]')
 ifeq ($(NDK_ABI),x86)
  HOST = i686-linux-android
@@ -44,11 +44,11 @@ else
  HOST = $(NDK_ABI)-linux-androideabi
  NDK_TOOLCHAIN = $(HOST)-$(NDK_TOOLCHAIN_VERSION)
 endif
-NDK_TOOLCHAIN_BASE=$(NDK_BASE)/toolchains/$(NDK_TOOLCHAIN)/prebuilt/$(NDK_UNAME)-$(NDK_PROCESSOR)
+NDK_TOOLCHAIN_BASE=$(ANDROID_NDK_HOME)/toolchains/$(NDK_TOOLCHAIN)/prebuilt/$(NDK_UNAME)-$(NDK_PROCESSOR)
 
 # include Android's build flags
 TARGET_ARCH_ABI = $(APP_ABI)
-include $(NDK_BASE)/toolchains/$(NDK_TOOLCHAIN)/setup.mk
+include $(ANDROID_NDK_HOME)/toolchains/$(NDK_TOOLCHAIN)/setup.mk
 
 
 # install root for built files

make-release-build

@@ -32,11 +32,11 @@ if [ -z $ANDROID_HOME ]; then
     fi
 fi
 
-if [ -z $ANDROID_NDK ]; then
+if [ -z $ANDROID_NDK_HOME ]; then
     if which ndk-build 2>&1 /dev/null; then
-        ANDROID_NDK=`which ndk-build |  sed 's,/ndk-build,,'`
+        ANDROID_NDK_HOME=`which ndk-build |  sed 's,/ndk-build,,'`
     else
-        echo "ANDROID_NDK not set and 'ndk-build' not in PATH"
+        echo "ANDROID_NDK_HOME not set and 'ndk-build' not in PATH"
         exit
     fi
 fi
@@ -59,7 +59,7 @@ git submodule foreach --recursive git submodule sync
 git submodule update --init --recursive
 
 make -C external/
-$ANDROID_NDK/ndk-build
+$ANDROID_NDK_HOME/ndk-build
 
 if [ -e ~/.android/ant.properties ]; then
     cp ~/.android/ant.properties $projectroot/