build-kvmtool: refactor output directory path

kvmtool is creating an output directory structure using the variable
"KVMTOOL_SUPPORT_PLATFORM" and this was set to "rdn2" regardless the
platform it was being built.

This change modifies the output path to place the 'lkvm' binary in
"output/<platform>/components/kvmtool" to align with other components.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Ied4b2f440cb0530cea450776a85c0df8929cffde