scripts: download-check-artifact.sh: fix shellcheck and improve

scripts: download-check-artifact.sh: fix shellcheck and improve

Fixes following shellcheck's recommendations:

 In scripts/download-check-artifact.sh line 24:
	exit $1
             ^-- SC2086 (info): Double quote to prevent globbing and word splitting.

 In scripts/download-check-artifact.sh line 53:
		local sum="$(shasum -a 256 "$image_file")";
                      ^-^ SC2155 (warning): Declare and assign separately to avoid masking return values.

 In scripts/download-check-artifact.sh line 72:
 cd "/tmp/verify.$$"
 ^-----------------^ SC2164 (warning): Use 'cd ... || exit' or 'cd ... || return' in case cd fails.

 In scripts/download-check-artifact.sh line 114:
		printf "Keyserver to use? [$keyserver_url] > "
                       ^-- SC2059 (info): Don't use variables in the printf format string. Use printf '..%s..' "$foo".

 In scripts/download-check-artifact.sh line 115:
		read url; case "${url:-$keyserver_url}" in
                ^--^ SC2162 (info): read without -r will mangle backslashes.

While at it make it clear, that it is possible to download/check any
build artifacts like even SDK or ImageBuilder.

Link: https://github.com/openwrt/openwrt/pull/16871
(cherry picked from commit 27c2c140b16cb490f20c1c2259b3decf50ae5762)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>