Extract GRUB config into separate template file

- Create config/grub.cfg.in template file for main GRUB configuration
- Update docker-image-create-uefi.sh to use template instead of heredoc
- Improves maintainability by keeping config separate from build logic
- Follows existing pattern used for grub-embedded.cfg.in

Co-authored-by: johndoe6345789 <224850594+johndoe6345789@users.noreply.github.com>

config/grub.cfg.in

@@ -0,0 +1,39 @@
+# GRUB Configuration for SparkOS
+
+# Load essential modules
+insmod part_gpt
+insmod fat
+insmod ext2
+insmod linux
+insmod all_video
+insmod video_bochs
+insmod video_cirrus
+insmod gfxterm
+
+# Set terminal and video modes
+terminal_input console
+terminal_output console
+set gfxmode=auto
+set gfxpayload=keep
+
+# Boot menu settings
+set timeout=5
+set default=0
+
+# Show countdown message
+echo "SparkOS Boot Menu - Starting in $timeout seconds..."
+
+menuentry "SparkOS" {
+    echo "Loading SparkOS kernel..."
+    linux /boot/vmlinuz root=LABEL=@ROOT_LABEL@ ro init=/sbin/init console=tty0 console=ttyS0,115200n8
+}
+
+menuentry "SparkOS (Verbose Mode)" {
+    echo "Loading SparkOS kernel in verbose mode..."
+    linux /boot/vmlinuz root=LABEL=@ROOT_LABEL@ ro init=/sbin/init console=tty0 console=ttyS0,115200n8 debug loglevel=7
+}
+
+menuentry "SparkOS (Recovery Mode)" {
+    echo "Loading SparkOS kernel in recovery mode..."
+    linux /boot/vmlinuz root=LABEL=@ROOT_LABEL@ rw init=/bin/sh console=tty0 console=ttyS0,115200n8
+}