Bluetooth: ISO: Move contents of Kconfig.iso to common Kconfig

Move the contents of Kconfig.iso to the common Kconfig
file.

The main reason for this is that the Kconfig.iso filename
has caused issues for some, due to the .iso filename extension.

The contents have been moved to right after the Kconfig options
for BT_CONN which are quite similar.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>