cmake: Rename main target library

Use a more descriptive name (arm-corstone-platform-bsp)
for the main CMake target library provided by the project.
The older name, `target-corstone300`, did not take into
account that platforms other than Corstone-300 will
be supported in the future.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>