Fix WKS_FILE to use files with .in extension

Fix WKS_FILE to use files with .in extension

meta-freescale commit 6be9d197386b5c3bd72023981df805d42f87684c
renamed imx-uboot-spl-bootpart.wks to imx-uboot-spl-bootpart.wks.in
The .in extension in wks files allows bitbake variables to be used in
kickstarter files. Set WKS_FILES for all machines to match this new
filename.

Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
(cherry picked from commit da4d8a203c173f8719229df5cb525b6ed644a7c6)