The building of the 'software/' directory will fail if the project
doesn't know the path to 'vme-bridge', 'fpga-mgr-backpoirt', 'fmc',
'i2c', and 'spi' projects.

Thus, to make the error more clear, let the build system directly point
out that a project is missing, instead of complaining for missing
headers.
Signed-off-by: Vaibhav Gupta <vaibhav.gupta@cern.ch>