image-fit: Fix FIT_CIPHER linking

When CONFIG_FIT_CIPHER=y and CONFIG_FIT_SIGNATURE=n is there is no
implementation of image_get_host_blob for mkimage/dumpimage:

 /usr/bin/ld: tools/common/image-cipher.o: in function `fit_image_decrypt_data':
 image-cipher.c:(.text+0x9a): undefined reference to `image_get_host_blob'

Move the implementation to a common file so it can be shaed between
image-cipher.c and image-fit-sig.c.

Signed-off-by: Joel Stanley <joel@jms.id.au>