uboot-phytec

arm: stm32mp1: deploy spl in root folder

Update generation of spl binaries
- continue to generate all SPL files in spl sub-directory
- copy in root folder the needed file for user (YOCTO, buildroot):
  u-boot-spl.stm32

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>