[mythtv] Proposed change to android build setenv.sh

[Peter Bennett]

The current build require setenv.sh to be copied to ~/android/.
Every time setenv.sh changes I have to copy it again and make my 
personalizations.
I propose add the following lines to the end of setenv.sh

if [[ -f $ANDROID_ROOT/custom.source ]] ; then
     . $ANDROID_ROOT/custom.source
fi

Then people can put their customizations in ~/android/custom.source.

Also I propose to change the scripts to source setenv.sh from 
MythTV/packaging/android/android-utilities/setenv.sh instead of from 
~/android.

My custom.source is:

export KEYSTORE=$HOME/.android/android-release-key.jks
export KEYALIAS=mythfrontend
export KEYSTOREPASSWORD=XXXXXXXX
# comment these for 32 bit build
cat <<EOF >make.inc
target_arch=arm64
ARM64=1
EOF

Changing between 32 bit and 64 bit involves only changing custom.source.

Any problems with this?  Maybe a different name for custom.source?

Peter