board/qemu: restore readme globbing

The QEMU board's post-image script will glob through various
`readme.txt` files for an appropriate command line to use. Recent linter
changes [1] prevents this from happening, and results in the following
build error:

    >>>   Executing post-image script board/qemu/post-image.sh
    sed: can't read board/qemu/*/readme.txt: No such file or directory

Reverting part of the shellcheck fixes for the README file arguments and
marking a linter exception.

[1]: b32d7c99c64d6dcb7a68036a7abfa86b8b558627

Signed-off-by: James Knight <james.d.knight@live.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: also reinstate plural to the variable name]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/qemu/post-image.sh

@@ -2,7 +2,7 @@
 
 QEMU_BOARD_DIR="$(dirname "$0")"
 DEFCONFIG_NAME="$(basename "$2")"
-README_FILE="${QEMU_BOARD_DIR}/*/readme.txt"
+README_FILES="${QEMU_BOARD_DIR}/*/readme.txt"
 START_QEMU_SCRIPT="${BINARIES_DIR}/start-qemu.sh"
 
 if [[ "${DEFCONFIG_NAME}" =~ ^"qemu_*" ]]; then
@@ -12,7 +12,8 @@ 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/#.*//' \ "${README_FILE}")"
+# shellcheck disable=SC2086 # glob over each readme file
+QEMU_CMD_LINE="$(sed -r ':a; /\\$/N; s/\\\n//; s/\t/ /; ta; /# '"${DEFCONFIG_NAME}"'$/!d; s/#.*//' ${README_FILES})"
 
 if [ -z "${QEMU_CMD_LINE}" ]; then
     # No Qemu cmd line found, can't test.