scripts: check_kernel_features.sh - allow overriding kernel config path
scripts: check_kernel_features.sh - allow overriding kernel config path

Allow check_kernel_features.sh to get passed the path to the kernel
config to look at instead of only probing a hard-coded list.

This allows to check the kernel configuration of a different but the
booted kernel as well.

Also make sure the file is readable and skip it otherwise.

Signed-off-by: Mathias Krause <64568d9183dcfcdd0a6fb47964820a1cdaf94ecc@grsecurity.net>

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace