linux: ensure correct make targets are used for cuImage/simpleImage.<dtb>

Fixes https://gitlab.com/buildroot.org/buildroot/-/jobs/66561794



LINUX_DTS_NAME may end up with a leading space because of the += logic, and
may contain multiple dts files - Neither of which works when we construct
the {cu,simple}Image.$(LINUX_DTS_NAME) make target name.

Fix it by using the first word in the variable.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>