Update old tunables framework document/script.

Since commit 8b9e9c3c, security_level replaces
is_secure. There were some old files need to be updated.

2017-03-23  Sunyeop Lee  <sunyeop97@gmail.com>

	* README.tunables: Updated descriptions.
	* elf/dl-tunables.list: Fixed typo: SXID_NONE -> NONE.
	* scripts/gen-tunables.awk: Updated the code related to the commit.