Adds a boot parameter which can be used to specify a delay (in seconds) before 
the root device is decoded/discovered/mounted.

Example usage for 10 second delay:

	rootdelay=10

Useful for usb-storage devices which no longer make their partitions 
immediately available, and for other storage devices which require some 
"spin-up" time.

Signed-off-by: Daniel Drake <dsd@gentoo.org>