fix dir/shield order

[Percentnineteen]

this is basically PR #12 updated to the file supported in zephyr 3.2 (the old one was 3.0 based, I believe). The edits look messy but that's just due to inverting of logic. There is nothing new here, just reorganization.

Without this PR, a shield in zmk-config cannot use the nice!view shield (or any other shields) defined in the main zmk dir.

Right now, the logic basically goes:
foreach location (zmk, zmk-config)
foreach shield
if shield exists, append it to the list

This results in a board list like "my_keyboard_shield nice_view" being inverted -> "nice_view my_keyboard_shield".

This PR just inverts the logic and does:
foreach shield
foreach location (zmk, zmk-config)
if shield exists, append it to the list

This still allows any shield defined in zmk-config to overwrite any shield also defined in zmk/apps/boards/shields, but also does not re-order the shields according to where they were found

[petejohanson]

LGTM. Thanks for the fix. Local testing here and the build seems sane.