common: Move helper scripts to a common directory

Consolidate all installation scripts into a common folder for better
file/folder organisation.
Move helper scripts required by dockerfiles into the common directory.

The new directory structure is as below.
setup-scripts/
├── common
│   ├── install-gcc.sh
|   |-- install-clang.sh
│   ├── install-libfdt.sh
│   └── install-openssl.sh
├── container-files
│   ├── rd-infra-amd64
│   └── rd-infra-arm64
├── container.sh

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: Ib25f14881f2c6e340b1f42b281e49bc2e9c69835