Add precompiled field to library.properties

A precompiled library should provide:
- a header file defining the functions (like a normal library)
- some folders named after {build.mcu} containing the precompiled files

It should declare
- precompiled=true in its library.properties

The precompiled files can be either static (.a) or dynamic (.so).
In the latter case, the maintainer should take extra care to ensure that the exported symbol versions are matching the remote board's

Author: facchinm

Diff for: src/arduino.cc/builder/constants/constants.go

@@ -137,6 +137,7 @@ const LIBRARY_LICENSE = "license"
 const LIBRARY_MAINTAINER = "maintainer"
 const LIBRARY_NAME = "name"
 const LIBRARY_PARAGRAPH = "paragraph"
+const LIBRARY_PRECOMPILED = "precompiled"
 const LIBRARY_PROPERTIES = "library.properties"
 const LIBRARY_SENTENCE = "sentence"
 const LIBRARY_URL = "url"

Diff for: src/arduino.cc/builder/libraries_loader.go

@@ -192,6 +192,7 @@ func makeNewLibrary(libraryFolder string, debugLevel int, logger i18n.Logger) (*
 	library.URL = strings.TrimSpace(libProperties[constants.LIBRARY_URL])
 	library.IsLegacy = false
 	library.DotALinkage = strings.TrimSpace(libProperties[constants.LIBRARY_DOT_A_LINKAGE]) == "true"
+	library.Precompiled = strings.TrimSpace(libProperties[constants.LIBRARY_PRECOMPILED]) == "true"
 	library.Properties = libProperties
 
 	return library, nil

Diff for: src/arduino.cc/builder/types/types.go

@@ -170,6 +170,7 @@ type Library struct {
 	Name          string
 	Archs         []string
 	DotALinkage   bool
+	Precompiled   bool
 	IsLegacy      bool
 	Version       string
 	Author        string