boot: add --append flag for extra kernel bootargs

[asrinivasanTT]

Adds the ability to extend the kernel commandline while doing bhx boot

[olofj]

No real concerns with this patch, with two minor comments:

1. --append is a bit generic, and it maps over to another name in the sources. It might be what tt-bh-linux uses though? --extra-bootargs would maybe be a better name. I'm not super fussed about it though, it can stay as it is.

2. The functions are taking tons of arguments in these areas of the code and this adds one more. I filed Cleanup sweep: simplify awkward internal APIs (param packs, redundant indices, leaky abstractions) #198 yesterday to clean up and refactor them. So this is just a heads up that there'll be some churn in those files soon to clean up the API, so this code will be touched as part of that.

Given (2) and to not hold things up on the refactor, I'll merge this now. Thanks for the PR!