scripts/functions: Fix CT_ExtractGit function.

Change CT_ExtractGit so that it clones the repository, instead of just
symlinking it. After cloning, any given ref is checked out, or if no
ref is given, the HEAD of the repository is checked out.

This makes CT_Extract behave similar for git repositories as it does
for tarballs, so that it for example can be used for passing glibc-ports
as a git repository.

Signed-off-by: "Esben Haabendal" <esben.haabendal@prevas.dk>
[yann.morin.1998@anciens.enib.fr: fix incomplete var rename]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Menu for the native GDB

 config GDB_NATIVE

     bool

     prompt "Native gdb"

     depends on ! BARE_METAL

     depends on ! BACKEND

     help

       Build and install a native gdb for the target, to run on the target.

 if GDB_NATIVE

 config GDB_NATIVE_STATIC

     bool

     prompt "Build a static native gdb"

     help

       In case you have trouble with dynamic loading of shared libraries,

       you will find that a static gdb comes in handy.

 endif # GDB_NATIVE