In /logs/firmware/unblob_extracted/firmware_extract/11108416-13869836.lzma_extract/lzma.uncompressed_extract/3761304-11775640.cpio_portable_ascii_extract/usr/bin/ap_update.sh line 13:

cd /tmp/ftp/

^----------^ SC2164 (warning): Use 'cd ... || exit' or 'cd ... || return' in case cd fails.

Did you mean: 

cd /tmp/ftp/ || exit

In /logs/firmware/unblob_extracted/firmware_extract/11108416-13869836.lzma_extract/lzma.uncompressed_extract/3761304-11775640.cpio_portable_ascii_extract/usr/bin/ap_update.sh line 16:

while [[ "$TIMEOUT" != 0 ]]

      ^-------------------^ SC3010 (warning): In POSIX sh, [[ ]] is undefined.

In /logs/firmware/unblob_extracted/firmware_extract/11108416-13869836.lzma_extract/lzma.uncompressed_extract/3761304-11775640.cpio_portable_ascii_extract/usr/bin/ap_update.sh line 19:

    if [ $? -ne 0 ]; then

         ^-- SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?.

In /logs/firmware/unblob_extracted/firmware_extract/11108416-13869836.lzma_extract/lzma.uncompressed_extract/3761304-11775640.cpio_portable_ascii_extract/usr/bin/ap_update.sh line 22:

	let "TIMEOUT-=1"

        ^-------------^ SC3039 (warning): In POSIX sh, 'let' is undefined.

For more information:

  https://www.shellcheck.net/wiki/SC2164 -- Use 'cd ... || exit' or 'cd ... |...

  https://www.shellcheck.net/wiki/SC3010 -- In POSIX sh, [[ ]] is undefined.

  https://www.shellcheck.net/wiki/SC3039 -- In POSIX sh, 'let' is undefined.