README.booting.md: Update and add notes
README.booting.md: Update and add notes

Remove mention of 'u-boot-dtb.img' which is no longer generated.

Add a small paragraph directing users to look at the Yocto dev manual
for information about generating partitioned/disk images with wic.

Signed-off-by: Nathan Rossi <2e8aa918660411855c6d44d5bb2da677aa033255@nathanrossi.com>
Reviewed-by: Alistair Francis <52ecf00b70a50079ae8d5fa09b9af6904950b63c@xilinx.com>

Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx