arm-bsp/rss/fvp-rd-kronos: Clean RSS NVM image file

When TF-M boots it verifies the authenticity the flash
using the NV Counters stored in the NVM Image.
But if the RSS flash file is recreated and the NVM image file
is not recreated, Both their contents do not align and the
RSS boot fails.

Fix this by removing and creating an empty RSS NVM image file
whenever the RSS Flash file is recreated.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>