install: Add support for Ubuntu 64-bit

Raised in [this forum post](https://discuss.bluerobotics.com/t/raspios-or-ubuntu-arm64/12236/4):

> `user: robins`
>
> In case anyone else is attempting to install on Ubuntu 20.04, here are a few solutions to problems you'll encounter.
>
> 1. rfkill isn't installed, which is easy to remedy.
> ```
> sudo apt-get install rfkill
> ```
> 2. The default user isn't named 'pi', which causes errors with the install script. Can be fixed by following this guide: [https://www.linuxuprising.com/2019/04/how-to-change-username-on-ubuntu-debian.html](https://www.linuxuprising.com/2019/04/how-to-change-username-on-ubuntu-debian.html)
> 3. A few file paths are different, which require you to clone the BlueOS repository locally and make necessary changes.
> - The path in [line 16](https://github.com/bluerobotics/BlueOS-docker/blob/0b6976f892bba2dfe5723b6a0373312c96dad22d/install/boards/bcm_27xx.sh#L16) of bcm_27xx.sh should be changed to:
> ```
> /boot/firmware/overlays/$DTS_NAME.dtbo
> ```
> - And [line 67](https://github.com/R-Stokke/BlueOS-docker/blob/1589dde77701fa702b6588a6a6ea9fd6ab27ea76/install/boards/bcm_27xx.sh#L67) to:
> ```
> /boot/firmware/cmdline.txt
> ```
> - And finally the paths in [lines 161 and 163](https://github.com/bluerobotics/BlueOS-docker/blob/886f486753ac68fbe092fb185ba505f3fcf5cba6/install/install.sh#L161) in the install.sh script should *both* be changed to:
> ```
> /etc/dhcp/dhcpd.conf
> ```
>
> I ended up opting to use 64-bit RaspiOS for my application, so **there may be more issues than are listed here**, but this might at least shave an hour or two off of your troubleshooting.

> `user: EliotBR`
>
> BlueOS is open source, and extendable to other systems - the best approach for this kind of thing is to do the modifications with checks to a variable (presumably `$ARCHITECTURE` in this case) as relevant, so that it chooses the correct code to execute for the system in use.