nanobsd: Increase default media size

[jlduran]

Summary

Increase the NANO_MEDIASIZE to ~7.6GiB (~8GB, 16,000,000 * 512B) in
order to accommodate a default build (with no configuration options).

[jlduran]

@imp expressed the possibility of implementing options similar to the ones in D43232 (https://github.com/freebsd/freebsd-src/pull/964#issuecomment-1872262987). While this is evidently an interesting option, I have opted to propose just a "bump" to the minimum default size. Default meaning having no *_OPTIONS.
At some point this will be inevitable, currently the command sh nanobsd.sh with no custom configuration, fails. Presumably someone creating a customized image would have options set accordingly. More importantly, it is likely that the NANO_MEDIASIZE is set to match the size of the physical card. Industrial cards with 8GB or more seem to be the norm, after a little market research.
At any rate, this is just a suggestion, and I will gladly implement the maintainer's recommendations. What is paramount for me would be to fix sh nanobsd.sh before asking doc for a review of D47634.

[imp]

I kinda think we should ditch this size altogether and nake minimal images by default. 8g is too big, imho, for a default that uses only 1/8 of that. Of we just use growfs and makefs/mkimg, we'll be good

[jlduran]

OK, I'll re-submit following that approach.

[imp]

I've wanted for years to have nanobsd not need root at all...

[jlduran]

In D47643#1086284, @imp wrote:

I've wanted for years to have nanobsd not need root at all...

Ah yes, I would like that now for OCI containers as well.