bsp: cvitek: update combine.sh for arm64

Move the combine.sh to under cv18xx_aarch64, so we can
run this script the same as scons and no need to change
cwd.

Rename the folder boot to prebuild just becuase all the
files under boot are prebuild binaries.
Move the prebuilt fip.bin to under prebuild/milkv-duo256m
because it's only for 256.

Update combine.sh accordingly and output fip.bin to the output
folder where we create fip.bin/boot.sd as other bsp.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

bsp/cvitek/cv18xx_aarch64/combine.sh

@@ -1,6 +1,7 @@
-C906_LITTLE_BIN_PATH=../../c906_little/rtthread.bin
-GEN_FIP_PATH=.
-DEPENDS_FILE_PATH=${GEN_FIP_PATH}/milkv-duo256m
+#!/bin/bash
+C906_LITTLE_BIN_PATH=../c906_little/rtthread.bin
+GEN_FIP_PATH=../output/milkv-duo256m/
+DEPENDS_FILE_PATH=./prebuild/milkv-duo256m
 
 BLCP_IMG_RUNADDR=0x05200200
 BLCP_PARAM_LOADADDR=0
@@ -20,8 +21,11 @@ if [ -f "$C906_LITTLE_BIN_PATH" ]; then
     BLCP_2ND_PATH=${C906_LITTLE_BIN_PATH}
 else
     echo "The file $C906_LITTLE_BIN_PATH does not exist. Execute scons to compile it."
+    exit
 fi
 
+mkdir -p $GEN_FIP_PATH
+
 echo "Combining fip.bin..."
 . ${DEPENDS_FILE_PATH}/blmacros.env && \
 ${DEPENDS_FILE_PATH}/fiptool.py -v genfip \