Simplify build scripts + misc fixes

With the introduction of binman-based builds [1], we can simplify our builds scripts to no longer depend on core-secdev-k3 nor on k3-image-gen.

Remove the no longer used variables to follow TI's instructions.

[1] https://git.ti.com/cgit/ti-u-boot/ti-u-boot/tree/doc/board/ti/am62x_sk.rst?h=ti-u-boot-2023.04#n116

Also:

• add -j to some make invocations where this was forgotten
• update release scripts to handle github and TI remotes a little better