Explicitly building sysroot about xargo

[japaric]

So that I could explicitly get xargo to build a sysroot by typing xargo --target={TARGET_NAME}.

That's the current behavior. It's just that xargo doesn't print to stdout by default (see #7). But if you use RUST_LOG you can see that's the case:

$ RUST_LOG=xargo xargo --target stm32f100                                                       <<<
INFO:xargo: target: stm32f100
INFO:xargo::sysroot: rustc is a nightly from: 2016-04-22
INFO:xargo::sysroot: fetching source tarball
INFO:xargo::sysroot: unpacking source tarball
INFO:xargo::sysroot: cross compiling crates
INFO:xargo::sysroot: calling: "cargo" "build" "--release" "--target" "stm32f100" "-p" "collections" "-p" "rand"
INFO:xargo::sysroot:
   Compiling core v0.0.0 (file:///home/japaric/.xargo/src/libcore)
   Compiling rustc_unicode v0.0.0 (file:///home/japaric/.xargo/src/librustc_unicode)
   Compiling rand v0.0.0 (file:///home/japaric/.xargo/src/librand)
   Compiling alloc v0.0.0 (file:///home/japaric/.xargo/src/liballoc)
   Compiling collections v0.0.0 (file:///home/japaric/.xargo/src/libcollections)

INFO:xargo::sysroot: symlinking host crates
error: Unknown flag: '--target'

Hmm, it errors at the end because xargo ends up calling cargo --target stm32f100. Perhaps we could special case xargo --target $foo to not call cargo.

[kent-mcleod]

Right. I don't see any potential problems with special casing that, unless in the future cargo --target $thing became a valid call.