Allow script parameters to be overridden by environment vars

[Philo]

I have a customised build.ps1 that allows certain script parameters to be overridden by environment variables, this is particularly useful for the -Verbosity parameter.

[CmdletBinding()]
Param(
    [string]$Script = $(if (Test-Path env:/CAKE_SCRIPT) { "$env:CAKE_SCRIPT" } else { "build.cake" }),
    [string]$Target = $(if (Test-Path env:/CAKE_TARGET) { "$env:CAKE_TARGET" } else { "Default" }),
    [string]$Configuration = $(if (Test-Path env:/CONFIGURATION) { "$env:CONFIGURATION" } else { "Release" }),
    [ValidateSet("Quiet", "Minimal", "Normal", "Verbose", "Diagnostic")]
    [string]$Verbosity = $(if (Test-Path env:/CAKE_VERBOSITY) { "$env:CAKE_VERBOSITY" } else { "Verbose" }),
    [switch]$Experimental,
    [Alias("DryRun","Noop")]
    [switch]$WhatIf,
    [switch]$Mono,
    [switch]$SkipToolPackageRestore,
    [Parameter(Position=0,Mandatory=$false,ValueFromRemainingArguments=$true)]
    [string[]]$ScriptArgs
)

I am by no means a "powershell guy" but thought that this sort of alteration would be beneficial to others.

Environment Variable Naming

CAKE_SCRIPT, CAKE_TARGET and CAKE_VERBOSITY are named to prevent potential conflicts with other environment variables, while CONFIGURATION is specifically named this way as AppVeyor (maybe other CI servers) will set this to the build configuration.

Of course I have only done this for the powershell bootstrapper as we are building on Windows, I assume a similar approach can be done for the .sh script.