Make it pass shellcheck by fixing obvious mistakes like '==' instead of
'='. Add "shellcheck disable" lines for things that are intentionally done
or would be a bit more complicated to rewrite. It would be nice to
refactor some things like '${outfile/initramfs-/uImage-}', but this should
be done in a separate change (patches welcome).

Remove unused variable "modules_path". While at it, add two new fatal
error messages instead of just "exit 1" without any comment and remove
redundant mkdir in APKBUILD.