tools: ko-release: make scripts bourne sh compliant [checkbashisms]

tools: ko-release: make scripts bourne sh compliant [checkbashisms]

$ checkbashisms ko-release-gen
possible bashism in ko-release-gen line 31 ('function' is useless):
function die {
possible bashism in ko-release-gen line 36 ('function' is useless):
function add_file {
possible bashism in ko-release-gen line 68 ('function' is useless):
function add_html_dir {
possible bashism in ko-release-gen line 72 (brace expansion):
        for fl in $(ls $src/*.{html,css,png}); do

$ checkbashisms ko-release-push
possible bashism in ko-release-push line 29 ('function' is useless):
function die {
possible bashism in ko-release-push line 34 ('function' is useless):
function push_file {

Signed-off-by: Sami Kerola <kerolasa@iki.fi>