k3: tools: config.mk: Update makefile and remove scripts

Since binman is used to package bootloader images for all K3 devices, we
do not have to rely on the earlier methods to package them.

Scripts that were used to generate x509 certificate for tiboot3.bin and
generate tispl.bin, u-boot.img have been removed.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>