expat XML parser library

CONFIG_LIB_EXPAT provides the Expat streaming XML parser as a reusable NuttX applications library.

General entity expansion and DTD processing are disabled by default. Enable CONFIG_LIB_EXPAT_GENERAL_ENTITIES or CONFIG_LIB_EXPAT_DTD only when an application requires them and its XML input policy has been reviewed.

Expat uses the MIT license, so CONFIG_ALLOW_MIT_COMPONENTS must be enabled.