Fixed merging western latitudes

When the 04-merge.sh script is called with negative longitudes, the wrong height files are being selected:

```
$ ./scripts/04-merge.sh North_America -125 -120 35 50
SRTM3/N35W125.hgt
SRTM3/N36E125.hgt
SRTM3/N37E125.hgt
SRTM3/N38E125.hgt
SRTM3/N39E125.hgt
```

Note the switch from W to E.  Moving the condition which sets the `$lon` and `$ew` variables in the script outside the latitude loop fixes this issue.