configs: rename rpi4_boot to rpi64_boot for consistency

Align bootloader naming with the board name raspberrypi-rpi64.

[skip ci]

Signed-off-by: Claude <noreply@anthropic.com>

.github/workflows/build-boot.yml

@@ -21,7 +21,7 @@ jobs:
           - cn9130_crb_boot
           - fireant_boot
           - nanopi_r2s_boot
-          - rpi4_boot
+          - rpi64_boot
     env:
       MAKEFLAGS: -j5
     steps:

.github/workflows/build-image.yml

@@ -54,7 +54,7 @@ jobs:
         run: |
           case "${{ inputs.board }}" in
             raspberrypi-rpi64)
-              echo "BOOTLOADER=rpi4_boot" >> $GITHUB_ENV
+              echo "BOOTLOADER=rpi64_boot" >> $GITHUB_ENV
               echo "TARGET=aarch64" >> $GITHUB_ENV
               ;;
             bananapi-bpi-r3)