binman: Fix blank line usage for invalid images warning text

binman: Fix blank line usage for invalid images warning text

There is no blank line between last missing blob help message and the
header line for optional blob help messages.

  Image 'simple-bin' is missing external blobs and is non-functional: atf-bl31

  /binman/simple-bin/fit/images/@atf-SEQ/atf-bl31:
     See the documentation for your board. You may need to build ARM Trusted
     Firmware and build with BL31=/path/to/bl31.bin
  Image 'simple-bin' is missing external blobs but is still functional: tee-os

  /binman/simple-bin/fit/images/@tee-SEQ/tee-os:
     See the documentation for your board. You may need to build Open Portable
     Trusted Execution Environment (OP-TEE) and build with TEE=/path/to/tee.bin

  Some images are invalid

With this a blank line is inserted to make the text more readable.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>