scripts: Address shellcheck issues

Report issues by shell files with:

    git ls-files '*.sh' | xargs shellcheck --exclude=SC2162

Address the following:

- SC1087: Use braces when expanding arrays
- SC1091: Not following
- SC2004: `$`/`${}` is unnecessary on arithmetic variables
- SC2024: `sudo` doesn't affect redirects
- SC2034: foo appears unused. Verify it or export it
- SC2086: Double quote to prevent globbing and word splitting
- SC2087: Quote `EOF`
- SC2115: Use `"${var:?}"` to ensure this never expands to `/*`
- SC2148: Add a shebang

Addresses (at least partially) some POSIX/dash issues:

- SC2113: `function` keyword is non-standard
- SC3010: In POSIX sh, `[[` `]]` is undefined
- SC3014: In POSIX sh, `==` in place of `=` is undefined
- SC3020: In POSIX sh, `&>` is undefined
- SC3046: In POSIX sh, `source` in place of `.` is undefined

Does not address:

- SC2162: `read` without `-r` will mangle backslashes
- Any other POSIX/dash-specific issues

Signed-off-by: Tim Crawford <tcrawford@system76.com>

scripts/_build/coreboot.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-if [ -z "$1" -o ! -e "$1" -o -z "$2" ]
+if [ -z "$1" ] || [ ! -e "$1" ] || [ -z "$2" ]
 then
   echo "$0 [coreboot.config] [coreboot.rom]" >&2
   exit 1
@@ -10,7 +10,7 @@ fi
 CONFIG="$(realpath "$1")"
 COREBOOT="$(realpath "$2")"
 
-function check_configs() {
+check_configs() {
   local defconfig="$1"
 
   while read -r line; do