main/postmarketos-mkinitfs: fix showing custom splash after running osk-sdl

Support for custom "Loading..." splashes was added in MR 1776, but
`unlock_root_partition` function is still hardcoding default "Loading..."
splash. Fix this behaviour by replacing usage of `show_splash` function with
`show_splash_loading`, which accounts for presence of custom splash.

Closes #759

...although most of the work was done in aforementioned MR 1776 :D

Untested because changes look simple enough to me