Fix bytes <-> kilobytes typo in Kconfig for ramdisk

This is a small fix for a typo in Kconfig. The default value for the block
size is 1024 bytes not 1024 kilobytes.

Signed-off-by: Christian Borntraeger <borntrae@de.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>