main: Display git commit hash with the '--version' option

Add a build-script to propagate the git commit hash to other crates at
compile time through environment variables, and display the hash along
with the '--version' option.

Fixes #729

Signed-off-by: Bo Chen <chen.bo@intel.com>

Cargo.toml

@@ -4,6 +4,7 @@ version = "0.5.0"
 authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"
 default-run = "cloud-hypervisor"
+build = "build.rs"
 
 [dependencies]
 arc-swap = ">=0.4.4"