package/libserial: bump to version 0.6.0rc2

Also:
  * Update patch #1
  * Remove patch #2, fixed upstream
  * add new dependency to boost
  * use hashes from sourceforge

[Thomas:
 - add locally calculated sha256 hash
 - show the Config.in comment about toolchain dependencies only when
   BR2_PACKAGE_BOOST_ARCH_SUPPORTS is set.]

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>