kernel.bbclass,linux.inc: cleanup definition of ${KERNEL_IMAGE_BASE_NAME}

kernel.bbclass,linux.inc: cleanup definition of ${KERNEL_IMAGE_BASE_NAME}
* drop ${DATETIME} from ${KERNEL_IMAGE_BASE_NAME} in kernel.bbclass, see:
  http://lists.linuxtogo.org/pipermail/openembedded-devel/2008-March/005017.html
* make ${KERNEL_IMAGE_BASE_NAME} and ${KERNEL_IMAGE_SYMLINK_NAME} ?=, see:
  http://lists.linuxtogo.org/pipermail/openembedded-devel/2008-March/005016.html
* remove ${KERNEL_IMAGE_BASE_NAME} and ${KERNEL_IMAGE_SYMLINK_NAME} from linux.inc
* use ${KERNEL_IMAGE_BASE_NAME} for uImage deploy