main/postmarketos-initramfs: show OS version on splash screen
This shows the OS version (from /etc/os-release), e.g. "edge", on the splash screen. os-release is a tiny text file, adding it to the initramfs doesn't seem too bad, and having this information on the splash screen can be helpful.
Note that this uses "VERSION" and not "PRETTY_NAME" from the os-release file, since the splash already shows "postmarketOS" (and the pretty name includes that too), it seemed redundant and took up valuable display space.