posix shell: use /bin/sh instead of bash

This uses the systems default shell:
- Ubuntu: dash
- Fedora: bash

Since bash is invoked via /bin/sh, it operates in POSIX mode:
  https://tiswww.case.edu/php/chet/bash/POSIX

- remove '# Ignore the expand_aliases command in zshell.'
  Not needed because the shell operates in POSIX mode
- [[ is bashism -> use [
- autostart_files=( $autostart_file_match )
  is not supported in dash, so use 'ls'
- shellcheck runs the dash flavor, since dash is a minimalistic shell.

Tested on dash & bash.

ROMFS/px4fmu_common/init.d-posix/rcS

@@ -5,7 +5,7 @@
 # shellcheck disable=SC1091
 . px4-alias.sh
 
-SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+SCRIPT_DIR="$(CDPATH='' cd -- "$(dirname -- "$0")" && pwd)"
 
 #
 # Main SITL startup script
@@ -149,10 +149,7 @@ then
 fi
 
 # Autostart ID
-autostart_file_match="etc/init.d-posix/$(param show -q SYS_AUTOSTART)_*"
-# shellcheck disable=SC2206
-autostart_files=( $autostart_file_match )
-autostart_file="${autostart_files[0]}" # use first match, but there should really only be one
+autostart_file=$( ls -- etc/init.d-posix/"$(param show -q SYS_AUTOSTART)"_* )
 if [ ! -e "$autostart_file" ]; then
 	echo "Error: no autostart file found"
 	exit -1