Almost everyone understands "rootfs", but "root filesystem"
is real English.

(From yocto-docs rev: 3d610f15e5b6be3435e9e81e4498a676671e5b39)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>