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/coreboot-gpio.sh

@@ -33,8 +33,8 @@ do
     do
         parts+=("$part")
     done
-    parts[1]="$(printf '0x%08x' "$((${parts[1]} & 0xfffffffd))")"
-    parts[2]="$(printf '0x%04x' "$((${parts[2]} & 0x00003c00))")"
+    parts[1]="$(printf '0x%08x' "$((parts[1] & 0xfffffffd))")"
+    parts[2]="$(printf '0x%04x' "$((parts[2] & 0x00003c00))")"
 
     case "${parts[1]}" in
         0x0???????)