board/qemu: fix shellcheck in post-image script

Commit 270759aff3cf (board/qemu: define start qemu script outside of
post-image script) touches board/qemu/post-image.sh, so that exposed
existing shellcheck issues.

Fix those now.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: James Knight <james.d.knight@live.com>

board/qemu/post-image.sh

@@ -1,8 +1,8 @@
 #!/bin/bash
 
-QEMU_BOARD_DIR="$(dirname $0)"
-DEFCONFIG_NAME="$(basename $2)"
-README_FILES="${QEMU_BOARD_DIR}/*/readme.txt"
+QEMU_BOARD_DIR="$(dirname "$0")"
+DEFCONFIG_NAME="$(basename "$2")"
+README_FILE="${QEMU_BOARD_DIR}/*/readme.txt"
 START_QEMU_SCRIPT="${BINARIES_DIR}/start-qemu.sh"
 
 if [[ "${DEFCONFIG_NAME}" =~ ^"qemu_*" ]]; then
@@ -12,7 +12,7 @@ fi
 
 # Search for "# qemu_*_defconfig" tag in all readme.txt files.
 # Qemu command line on multilines using back slash are accepted.
-QEMU_CMD_LINE=$(sed -r ':a; /\\$/N; s/\\\n//; s/\t/ /; ta; /# '${DEFCONFIG_NAME}'$/!d; s/#.*//' ${README_FILES})
+QEMU_CMD_LINE="$(sed -r ':a; /\\$/N; s/\\\n//; s/\t/ /; ta; /# '"${DEFCONFIG_NAME}"'$/!d; s/#.*//' \ "${README_FILE}")"
 
 if [ -z "${QEMU_CMD_LINE}" ]; then
     # No Qemu cmd line found, can't test.