package/perf: migrate perf to use the new linux-tools infrastructure

Remove the perf package and add legacy handling.

[Thomas:
  - improve the Config.in.legacy help text
  - improve the comment explaining why we pass O= when building perf]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>