gnu: icedtea6, icedtea7: Unpack to "openjdk.src".

* gnu/packages/java.scm (icedtea6)[arguments]: Unpack OpenJDK sources to
"openjdk.src".
(icedtea7)[arguments]: Inherit "--with-openjdk-src-dir" flag; refer to
"openjdk.src" in build phases; simplify "unpack" phase.
[native-inputs]: Rename "openjdk-drop" to "openjdk-src".