src/.../Kconfig: various small fixes to texts

Fixed spelling and added empty lines to separate the help
from the text automatically added during make menuconfig.

Change-Id: I6eee2c86e30573deb8cf0d42fda8b8329e1156c7
Signed-off-by: Daniele Forsi <dforsi@gmail.com>
Reviewed-on: http://review.coreboot.org/6313
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>