scripts: avoid global variables in target handler scripts

scripts/clean_output.lua

scripts/copy_output.lua

scripts/generate_manifest.lua

scripts/target_config.inc.lua

scripts/target_config.lua

scripts/target_config_check.lua

scripts/target_config_lib.lua

scripts/target_lib.lua

targets/brcm2708-bcm2708

targets/brcm2708-bcm2709

targets/brcm2708-bcm2710

targets/x86-64

targets/x86-generic