Inspired from answers by @i4niac & @Aurélien Lambert, this is what i came up with
csv_update_numbers=$(./android list sdk --all | grep 'Android SDK Build-tools' | grep -v 'Obsolete' | sed 's/\(.*\)\- A.*/\1/'|sed '/^$/d'|sed -e 's/^[ \t]*//'| tr '\n' ',')
csv_update_numbers_without_trailing_comma=${csv_update_numbers%?}
( sleep 5 && while [ 1 ]; do sleep 1; echo y; done ) \
| ./android update sdk --all -u -t $csv_update_numbers_without_trailing_comma
Explanation
android list sdk --all command (Ignoring obsolete packages).