init: wait up to 3s for USB block device before logging

The USB boot device takes time to appear in devtmpfs.
Without waiting, init logging silently fails and we get
no diagnostic output.

Co-Authored-By: Claude Opus 4.6 (1M context) <[e-mail]>

fedac/native/initramfs/init

···
 mount -t efivarfs efivarfs /sys/firmware/efi/efivars 2>/dev/null
 
 # Early USB mount for init logging (before ac-native starts)
+# Wait up to 3s for block devices to appear
 INIT_LOG=""
-for usbdev in /dev/sda1 /dev/sdb1 /dev/sdc1; do
-    if [ -b "$usbdev" ]; then
-        mkdir -p /mnt/usb
-        mount -o rw "$usbdev" /mnt/usb 2>/dev/null && {
-            INIT_LOG="/mnt/usb/ac-init.log"
-            echo "=== AC Init $(date 2>/dev/null || echo 'boot') ===" > "$INIT_LOG"
-            break
-        }
-    fi
+USB_WAIT=0
+while [ $USB_WAIT -lt 30 ]; do
+    for usbdev in /dev/sda1 /dev/sdb1 /dev/sdc1; do
+        if [ -b "$usbdev" ]; then
+            mkdir -p /mnt/usb
+            mount -o rw "$usbdev" /mnt/usb 2>/dev/null && {
+                INIT_LOG="/mnt/usb/ac-init.log"
+                echo "=== AC Init $(date 2>/dev/null || echo 'boot') ===" > "$INIT_LOG"
+                break 2
+            }
+        fi
+    done
+    sleep 0.1
+    USB_WAIT=$((USB_WAIT + 1))
 done
 # Log helper — writes to USB if mounted, always to kmsg
 ilog() {