uname: add -p (platform) support as fixed string 'unknown'

[frikiluser]

A kind of dummy option required by some building scripts.

I've found a common output for uname -p is "unknown" in Linux boxes. I can't guarantee this is the expected output for all Toybox supported platforms.

Some usage examples here:

• https://codesearch.debian.net/search?q=uname.*-p&literal=0
• https://codesearch.debian.net/show?file=pd-beatpipe_0.1-6%2FMakefile&line=70 (Darwin example expecting arm?)
• https://sources.debian.org/src/filtergen/0.12.8-1/debian/patches/debian-changes/ (Usage on *BSD and other Unix platforms where expects specific strings other than unknown)

Hope the code follows style recommendations. It's my first chunk of code for Toybox, that's why I've chosen to add a simple option that solves some situations on building key-packages.

Regards,

[landley]

Commit a9bd5923aa1a

[landley]

Closing in the absence of complaint about the commit.