main/postmarketos-mkinitfs: improve keyboard-font check
Fix failure in the osk-sdl "keyboard-font" check when there is a second key starting with "keyboard-font" (as in osk-sdl!105 (merged)):
==> initramfs: creating /boot/initramfs-postmarketos-allwinner-extra
ERROR: Font specified in /etc/osk.conf does not exist!
ERROR: postmarketos-mkinitfs-0.17-r0.trigger: script exited with error 1