Add "un/install dev target for linux" to Makefile

bitmask-root needs to be in in the path (/usr/local/bin did not work). Maybe I overlook something here, but this seems required to get the dev environment up and running.
This is currently how I run/debug things:

make vendor (required only once)
make build && make install_dev_linux && DEBUG=1 LEAP_DRYRUN=1 build/qt/release/riseup-vpn --no-systray

@jkito