build-ndk.sh use basename to determine ndk version; update readme for onionmasq-mobile var
- improved build-ndk.sh's ndk version detection from using a bash patern match which failed if the env var directory had a trailing slash, to use
basename
which works in both cases - updated the readme with an extra var needed by onionmasq-mobile, and a note that the _home var won't override if the _root var is set