kernel: add KERNEL_FTRACE_SYSCALLS option

Newer kernels propose to trace system calls, add an option to allow just
that.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37146 3c298f89-4303-0410-b956-a3cf2f4a3e73