initramfs-framework: overlayroot: fix kernel commandline clash

initramfs-framework: overlayroot: fix kernel commandline clash

The initramfs-framwork takes kernel commandline options and converts
them for internal use to variables following the 'bootparam_name'
pattern. The overlayroot module uses 'bootparam_rootfstype' and
'bootparam_rootfsflags' which both collide with linux kernel
parameters 'rootfstype' and 'rootfsflags'.  This collision is solved
by putting the variables in the namespace of the initramfs-framework
module.

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