Allow to set dependencies from user's system

Currently it is impossible to set something like
  llvmHome.linux_x64 = /usr
because DependencyProcessor has only `resolveRelative` method
which looks only inside dependencies folder.
Introduction of less restrictive `resolve` method solves this problem.

dependencies/build.gradle

@@ -98,7 +98,7 @@ platformManager.filteredOutEnabledButNotSupported.each { target ->
     DependencyKind.values().each { kind ->
         def dir = kind.getDirectory(loader)
         if (dir != null) {
-            String path = dependencyProcessor.resolveRelative(dir).canonicalPath
+            String path = dependencyProcessor.resolve(dir).canonicalPath
             rootProject.ext.set(kind.getPropertyName(target), path)
         }
     }

shared/src/main/kotlin/org/jetbrains/kotlin/konan/target/KonanProperties.kt

@@ -58,7 +58,7 @@ abstract class KonanPropertiesLoader(override val target: KonanTarget,
         = properties.hostTargetList(key, target, host)
 
     override fun absolute(value: String?): String =
-            dependencyProcessor!!.resolveRelative(value!!).absolutePath
+            dependencyProcessor!!.resolve(value!!).absolutePath
     private val dependencyProcessor  by lazy {
         baseDir?.let {
             DependencyProcessor(

shared/src/main/kotlin/org/jetbrains/kotlin/konan/util/DependencyProcessor.kt

@@ -249,7 +249,22 @@ class DependencyProcessor(dependenciesRoot: File,
         }
     }
 
-    fun resolveRelative(relative: String): File {
+    /**
+     * If given [path] is relative, resolves it relative to dependecies directory.
+     * In case of absolute path just wraps it into a [File].
+     *
+     * Support of both relative and absolute path kinds allows to substitute predefined
+     * dependencies with system ones.
+     *
+     * TODO: It looks like DependencyProcessor have two split responsibilities:
+     *  * Dependency resolving
+     *  * Dependency downloading
+     *  Also it is tightly tied to KonanProperties.
+     */
+    fun resolve(path: String): File =
+            if (Paths.get(path).isAbsolute) File(path) else resolveRelative(path)
+
+    private fun resolveRelative(relative: String): File {
         val path = Paths.get(relative)
         if (path.isAbsolute) error("not a relative path: $relative")