initramfs-framework: overlayroot: align bootparams with module name

initramfs-framework: overlayroot: align bootparams with module name

Renaming the 'rootrw' kernel commandline parameter to
'overlayrootrwdev' to both align better with this modules name, and
point out the usage of the variable.

This patch also includes an if block to migrate the old 'rootrw'
block, should it be already used by someone.

Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>