Integrating new package issue for generic image build

[shlomibitton]

Description
Integrating new package into the sonic-buildimage system is not working for generic image.
The new package is not a debian based, It is for building a binary and copy the output to the relevant locations.
All other images were successfully built.
The issue can be observed on this PR: #5725

Steps to reproduce the issue:

1. Try to build using the PR Backport ethtool to support QSFP-DD #5725

Describe the results you received:

sudo cp target/files/buster/ethtool ./fsroot/usr/bin
cp: cannot stat 'target/files/buster/ethtool': No such file or directory
sudo LANG=C chroot ./fsroot umount /proc
true
[ FAIL LOG END ] [ target/sonic-generic.bin ]
make: *** [slave.mk:824: target/sonic-generic.bin] Error 1
make[1]: *** [target/sonic-generic.bin] Error 2

Describe the results you expected:
Image should successfully build out.