main/postmarketos-mkinitfs: improve keyboard-font check (MR 1782)

Fix failure in the osk-sdl "keyboard-font" check when there is a second
key starting with "keyboard-font" (as in osk-sdl MR 105):

	==> 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