Update:
 - Python 3.6 support fixed. It is now correctly compiled and available.

Fixes:
 - disabled context and fiber for unsupported targets

There are several architectures which are not supported by context nor
 fiber. Because of this, Boost build was failing targets. This created the
 situation where the other boost libs were not being built and consequently,
 becoming unavailable  at the OpenWRT/LEDE repositories.
To solve this issue, it was necessary to disabled fiber and context for
 the incompatible targets.

Boost.Fiber is disabled for:
 -> ar7, rb532, brcm63xx.smp, brcm63xx, brcm47xx, brcm47xx.legacy, brcm2708,
    au1000, ath25, adm8668 and adm5120.

Boost.Context is disabled for:
 -> avr32, octeon and netlogic.

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>