package/boost: fixup Optimization flag in boost build

package/boost: fixup Optimization flag in boost build

When building with Boost Build the CXXFLAGS are extended depending
on the optimization level set. When not defined explicitly the
optimization level depends on the <variant>. For release it's 'speed'
and for debug it's set to 'off'

These flags overwrite the -O flag passed in with TARGET_CXXFLAGS as
it is appended when calling g++.

This commit sets the Optimization flags generated by Boost Build
to the value of TARGET_OPTIMIZATION no matter what level is used.

As Boost Build offers no nice way to alter those values the gcc
toolchain file is altered directly.

Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>