[AVR32] Drop support for redundant "keepinitrd" boot-time parm.

Given the existing "retain_initrd" boot-time parameter defined in
init/initramfs.c, there appears to be no need for the equivalent
"keepinitrd" parameter.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>