add --skip-sync startup flag

This adds a new startup flag `--skip-sync`. It will force spacemacs to skip
package synchonization. This can be useful in cases when you're working under
poor or restrictive network.

Thanks, @zaript, for this idea!

core/core-command-line.el

@@ -16,6 +16,9 @@
 (defvar spacemacs-insecure nil
   "If non-nil force Spacemacs to operate without secured protocols.")
 
+(defvar spacemacs-sync-packages t
+  "If non-nil synchronize packages on startup")
+
 (defun spacemacs//parse-command-line (args)
   "Handle Spacemacs specific command line arguments.
 The reason why we don't use the Emacs hooks for processing user defined
@@ -53,6 +56,8 @@ arguments is that we want to process these arguments as soon as possible."
                  i (1+ i)))
           ("--resume-layouts"
            (setq spacemacs-force-resume-layouts t))
+          ("--no-sync"
+           (setq spacemacs-sync-packages nil))
           (_ (push arg new-args))))
       (setq i (1+ i)))
     (nreverse new-args)))

core/core-configuration-layer.el

@@ -436,9 +436,9 @@ refreshed during the current session."
       (package-read-all-archive-contents)
       (unless quiet (spacemacs-buffer/append "\n")))))
 
-(defun configuration-layer/sync (&optional no-install)
+(defun configuration-layer/sync (&optional install)
   "Synchronize declared layers in dotfile with spacemacs.
-If NO-INSTALL is non nil then install steps are skipped."
+If INSTALL is non nil then install missing packages."
   (run-hooks 'configuration-layer-pre-sync-hook)
   (dotspacemacs|call-func dotspacemacs/layers "Calling dotfile layers...")
   (setq dotspacemacs--configuration-layers-saved
@@ -461,7 +461,7 @@ If NO-INSTALL is non nil then install steps are skipped."
   ;; load layers lazy settings
   (configuration-layer/load-auto-layer-file)
   ;; install and/or uninstall packages
-  (unless no-install
+  (when install
     (let ((packages
            (append
             ;; install used packages

init.el

@@ -27,7 +27,7 @@
                      "core/core-load-paths.el"))
   (require 'core-spacemacs)
   (spacemacs/init)
-  (configuration-layer/sync)
+  (configuration-layer/sync spacemacs-sync-packages)
   (spacemacs-buffer/display-startup-note)
   (spacemacs/setup-startup-hook)
   (require 'server)