RFC: linux-sony-nicki: Add CFLAGS to optimize the kernel binary

I marked the MR as RFC due to my concerns about the implementation not being ideal, though I think using deviceinfo isn't a terrible idea either.

I'll paste the commit message as it's quite self-explanatory:

As the kernel is only used by one device, it won't break anything for others. This gives the phone a free (minor) performance boost, but as it isn't very powerful, every improvement is welcome.

Flags were set based on reference detailed on the wiki page: https://wiki.postmarketos.org/wiki/Optimizing_the_kernel_binary