escaping $ in build_flags

[jschoch]

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Configuration

Operating system: windwos

PlatformIO Version (platformio --version):

Description of problem

using env.StringifyMacro() doesn't escape special characters also it isn't obvious what the escapes are, for instance "$$" to escape "$" isn't obvious. "\$" produces "/$" which is also baffeling. The docs only say to escape characters they don't say how you do it.

Request: document escape characters and special characters used in .ini build_flags and env variables