package/bitcoin: new package

package/bitcoin: new package

Bitcoin Core is an open source project which maintains and releases
Bitcoin client software called “Bitcoin Core”.

Signed-off-by: Fabio Urquiza <fabiorush@gmail.com>
[Thomas:
 - Don't create a new blockchain applications sub-menu for now, put
   this package in "Miscellaneous applications"
 - Do not select BR2_INSTALL_LIBSTDCPP, use depends on instead, and
   add the corresponding comment.
 - Do not select BR2_TOOLCHAIN_BUILDROOT_USE_SSP. Instead pass
   --disable-hardening, and let Buildroot pass the appropriate CFLAGS
   when hardening features are enabled system-wide.
 - Add missing BR2_TOOLCHAIN_HAS_ATOMIC dependency
 - Add quirky !(BR2_arm || BR2_armeb) || BR2_USE_MMU because the
   Cortex-M toolchains don't provide 8-byte __atomic intrinsics, but
   we don't have a good way to express that today
 - Add missing BR2_TOOLCHAIN_HAS_GCC_BUG_64735 due to the use of
   std::future
 - Use only one BITCOIN_CONF_OPTS assignment to pass all options]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>