Missing headers while trying to compile

Hi everyone, this is my first attempt trying to compile in openWRT.

*I did not use the precompiled SDK. I followed the instructions and prepared the build system:

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cd /home/buildbot/source
git checkout v17.01.2
make clean
./scripts/feeds update -a
./scripts/feeds install -a
make menuconfig
make toolchain/install
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*I used my own dir names and checkout versions...
I checked out 19.07.7.

I started reading at https://openwrt.org/docs/guide-developer/helloworld/chapter1 and kept reading up to chapter 6 (Migrating to use GNU make in your application).

Everything went smooth without any errors.
I created the make file and compiled my first program without errors but there were a few dependency warnings like this:
WARNING: Makefile 'package/feeds/packages/ksmbd/Makefile' has a dependency on 'kmod-crypto-arc4', which does not exist
Not all of them were referring to 'kmod-crypto-arc4' though!

So, i want to use lot's of different header files and i keep getting "no such file" errors.

Should i have selected more items in the menuconfig ???
The only thing i selected (as per instructions) was the target arc.

I am willing to use opencv to capture video frames from usb cams connected to the HLK-7628N board and create a server with socket/bind/listen/accept that sends video frames to a client.

Is opencv in the SDK? Will i be able to use it?

Any help will be highly appreciated.

Thanks
Manos