common/install-*.sh : Standardise installation scripts

Install helper scripts were written at different times, with varying
argument patterns, variable naming, etc. Making them have common
calling argument patterns improves their readbility and usage.
Update the naming convention for common variables and temporary paths.
Also add usage functions to print usage help string.

- common/install-openssl.h
- common/install-gcc.h
- common/install-clang.h
- common/install-libfdt.h

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: I889a91bf4aaf2f81d090c549aa489e8df2df7926