.gdbinit: load QEMU sub-commands when gdb starts

The scripts/qemu-gdb.py file is not easily discoverable.  Add a .gdbinit
file so GDB either loads qemu-gdb.py automatically or prints a message
informing the user how to enable them (some systems disable ./.gdbinit
loading for security reasons).

Symlink .gdbinit and the scripts directory in order to make out-of-tree
builds work.  The scripts directory is used to find the qemu-gdb.py file
specified by a relative path in .gdbinit.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NEric Blake <eblake@redhat.com>
Message-id: 20170517124042.1430-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

.gdbinit

+# GDB may have ./.gdbinit loading disabled by default.  In that case you can
+# follow the instructions it prints.  They boil down to adding the following to
+# your home directory's ~/.gdbinit file:
+#
+#   add-auto-load-safe-path /path/to/qemu/.gdbinit
+
+# Load QEMU-specific sub-commands and settings
+source scripts/qemu-gdb.py

configure

@@ -6376,6 +6376,7 @@ FILES="$FILES pc-bios/spapr-rtas/Makefile"
 FILES="$FILES pc-bios/s390-ccw/Makefile"
 FILES="$FILES roms/seabios/Makefile roms/vgabios/Makefile"
 FILES="$FILES pc-bios/qemu-icon.bmp"
+FILES="$FILES .gdbinit scripts" # scripts needed by relative path in .gdbinit
 for bios_file in \
     $source_path/pc-bios/*.bin \
     $source_path/pc-bios/*.lid \