Update build docs and s2e-config-template.lua files in examples

[LJP-TW]

After #11 , I update the build docs to avoid people going the wrong way like me.

The paths in the s2e-config-template.lua files in examples are hardcoded, this PR also fix this problem.

For example:

add_plugin("HostFiles")
pluginsConfig.HostFiles = {
    baseDirs = {
        "/home/aesophor/s2e/projects/sym_stdin"
    },
    allowWrite = true,
}

The code above is updated to:

add_plugin("HostFiles")
pluginsConfig.HostFiles = {
    baseDirs = {
        os.getenv("HOME") .. "/s2e/projects/sym_stdin"
    },
    allowWrite = true,
}

[aesophor]

OMG Thanks so much!!!