ti-sgx-km: rename options to have proper prefix

The sub-options of the ti-sgx-km package had their name option
prefixed by BR2_PACKAGE_TI_SGX, while the prefix should be
BR2_PACKAGE_TI_SGX_KM. This commit fixes that, and adds the necessary
Config.in.legacy handling.

Since those options are part of a choice, the legacy handling cannot
select the new name of the options, so the legacy handling only
informs the user of the rename.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>