kernel: move the leading dash into KERNEL_ARTIFACT_NAME

kernel: move the leading dash into KERNEL_ARTIFACT_NAME

* this matches how IMAGE_MACHINE_SUFFIX works and we can use
  that for the default value

* allows to set IMAGE_MACHINE_SUFFIX to empty for people
  who prefer to keep MACHINE name only in the directory name
  otherwise there would be a stray dash in:
  lrwxrwxrwx 2 martin martin   12 Nov 18 13:25 bzImage -> bzImage-.bin
  -rw-r--r-- 2 martin martin  12M Nov 18 13:25 bzImage-.bin
  -rw-r--r-- 2 martin martin 182M Nov 18 13:25 modules-.tgz
  if you set
  KERNEL_ARTIFACT_NAME = ""

[YOCTO #12937]

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>