convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

elfloader-tool: explicitly link config libs

explicitly link kernel_autoconf and elfloader_Config
since the global config lib is going to be remove

Fix licenses

elfloader: Stop using SEL4_ARCH for selective compilation in Makefile