os-release: add os-release-initrd package

os-release: add os-release-initrd package

When systemd is used inside an initramfs, it uses the presence of
`/etc/initrd-release` to detect when the system is in the initrd phase.
Otherwise `/etc/initrd-release` has the same format as
`/etc/os-release`.

Add `os-release-initrd` package to provide `/etc/initrd-release` as a
symlink to the os-release file. To avoid adding this file to the
`os-release` package, explicitly only add `/etc/os-release` to the
`os-release` package.

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>