fix: correct Ubuntu/macOS host_os detection via uname -s

[idootop]

Problem

On older macOS versions (e.g., macOS 11), uname -o is unsupported (BSD uname lacks this GNU option). This causes host OS detection to fail.

Additionally, the flash script inconsistently uses uname -o on Ubuntu, even though uname -s is the more reliable method across platforms.

Relative issue:

• aml-flash-tool on Linux  #30

Solution

• ​​macOS​​: Fall back to uname -s (outputs Darwin) when -o is unavailable.
• Ubuntu​​: Standardize on uname -s (outputs Linux) instead of uname -o (GNU/Linux) for consistency.

[idootop]

@numbqq cc

[viraniac]

Additionally, the flash script inconsistently uses uname -o on Ubuntu, even though uname -s is the more reliable method across platforms.

Thanks for catching this. When I made the change, I did try to verify that I am getting the correct output, but in the hurry I used a alpine container to check uname -o and it did gave Linux as output. However I see now uname -o output is not consistent across different distributions