Quote integer variables for consistency and safety

This adds quotes around all integer variable usages to follow defensive
programming best practices:
- Quote variable assignments: ret="$?"
- Quote in numerical comparisons: [ "$ret" -eq 0 ]
- Quote in array indexing: ${MESSAGES["$ret"]}
- Quote in exit statements: exit "$ret"

While unquoted variables work in numeric contexts, quoting ensures:
1. Consistent coding style across the codebase
2. Protection against unexpected word splitting
3. Better shellcheck compliance
4. Clearer intent that these are variable expansions

Author: jserv

.ci/autorun.sh

@@ -12,18 +12,18 @@ expect "# " { send "uname -a\n" } timeout { exit 2 }
 expect "riscv32 GNU/Linux" { send "\x01"; send "x" } timeout { exit 3 }
 DONE
 
-ret=$?
+ret="$?"
 
 MESSAGES=("OK!" \
      "Fail to boot" \
      "Fail to login" \
      "Fail to run commands" \
 )
 
-if [ $ret -eq 0 ]; then
-    print_success "${MESSAGES[$ret]}"
+if [ "$ret" -eq 0 ]; then
+    print_success "${MESSAGES["$ret"]}"
 else
-    print_error "${MESSAGES[$ret]}"
+    print_error "${MESSAGES["$ret"]}"
 fi
 
-exit ${ret}
+exit "$ret"

.ci/test-netdev.sh

@@ -58,7 +58,7 @@ for NETDEV in "${NETWORK_DEVICES[@]}"; do
     TEST_NETDEV "$NETDEV"
 done
 
-ret=$?
+ret="$?"
 
 MESSAGES=("OK!" \
      "Fail to boot" \
@@ -67,10 +67,10 @@ MESSAGES=("OK!" \
      "Fail to transfer packet" \
 )
 
-if [ $ret -eq 0 ]; then
-    print_success "${MESSAGES[$ret]}"
+if [ "$ret" -eq 0 ]; then
+    print_success "${MESSAGES["$ret"]}"
 else
-    print_error "${MESSAGES[$ret]}"
+    print_error "${MESSAGES["$ret"]}"
 fi
 
-exit ${ret}
+exit "$ret"