SCons: Utilize native Variable alias functionality

[Repiteo]

Instead of making platform and p separate variable options, we can instead make use of SCons' native alias functionality by putting both options together in a list. This makes for a slightly prettier --help output & allowed simplifying some selected_platform conditionals.

Before:

platform: Target platform (web|windows)
    default: 
    actual: windows

p: Platform (alias for 'platform')
    default: 
    actual:

After:

platform: Target platform (web|windows)
    default:
    actual: windows
    aliases: ['p']

[Riteo]

You're a SCons wizard :)

[akien-mga]

Neat, thanks for looking that up!

[akien-mga]

Thanks!