Flashing the BIOS on a DL380 G5 without Windows
Solution 1:
Use the HP Service Pack for ProLiant. It's a bootable DVD image, which will upgrade all the firmware on your server, including the BIOS.
If you don't have a DVD drive, or don't want to use one, you can use the HP USB Key Utility on a windows computer to create a bootable USB key from the image.
A third choice is to use the iLO console, and mount the image there. This would be the preffered choice, unless you're already standing right next to the server.