bctoolbox: remove redundant mbedtls dependency

Fixes:

  Makefile:532: *** mbedtls is in the dependency chain of bctoolbox that
  has added it to its _DEPENDENCIES variable without selecting it or
  depending on it from Config.in.  Stop.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>