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.sh

@@ -46,7 +46,7 @@ EDK2_ARGS+=(
 )
 
 # Rebuild gop-policy (used by edk2)
-if [ -e "${MODEL_DIR}/IntelGopDriver.inf" -a -e "${MODEL_DIR}/vbt.rom" ]
+if [ -e "${MODEL_DIR}/IntelGopDriver.inf" ] && [ -e "${MODEL_DIR}/vbt.rom" ]
 then
     touch apps/gop-policy/Cargo.toml
     FIRMWARE_OPEN_VBT="${MODEL_DIR}/vbt.rom" \
@@ -84,7 +84,7 @@ KERNELVERSION="${VERSION}" \
         "${COREBOOT}"
 
 # Rebuild EC firmware for System76 EC models
-if [ ! -e  "${MODEL_DIR}/ec.rom" -a -e "${MODEL_DIR}/ec.config" ]
+if [ ! -e  "${MODEL_DIR}/ec.rom" ] && [ -e "${MODEL_DIR}/ec.config" ]
 then
     env VERSION="${VERSION}" \
         ./scripts/_build/ec.sh \