Remove dashes from image names: 'BL3-x' --> 'BL3x'

This patch removes the dash character from the image name, to
follow the image terminology in the Trusted Firmware Wiki page:

    https://github.com/ARM-software/arm-trusted-firmware/wiki

Changes apply to output messages, comments and documentation.

non-ARM platform files have been left unmodified.

Change-Id: Ic2a99be4ed929d52afbeb27ac765ceffce46ed76
diff --git a/plat/arm/css/common/css_common.mk b/plat/arm/css/common/css_common.mk
index 8fcfbe5..aabcb46 100644
--- a/plat/arm/css/common/css_common.mk
+++ b/plat/arm/css/common/css_common.mk
@@ -56,7 +56,7 @@
 endif
 
 ifneq (${RESET_TO_BL31},0)
-  $(error "Using BL3-1 as the reset vector is not supported on CSS platforms. \
+  $(error "Using BL31 as the reset vector is not supported on CSS platforms. \
   Please set RESET_TO_BL31 to 0.")
 endif