Make flat-nodes an extra build option

[dbahrdt]

Currenty the user has to specify a flat-nodes file in OSM2PGSQL_EXTRA_ARGS. This however is quite cumbersome and produces issues if the flag is not set during updates but was set during import. Instead we should add an option to enable flat-nodes with a fixed file name (i.e. /nodes/flatnodes.bin). We can then store this info for the update script.

[dbahrdt]

Maybe we should get rid of OSM2PGSQL_EXTRA_ARGS all together or at least check the args for flags that should be set using env variables.

Cache size comes to mind (-C) and the number of threads.

[dbahrdt]

Better use /nodes/data/flatnodes.bin so that we can chown data in case /nodes is a filesystem root.