boost: slightly reduce the verbosity of the build

At the moment, the boost build is very verbose, it gives both the
Jam-level command being executed, and the underlying system command
being executed, with lots of newlines. Makes it hard to see where the
failure is when there is one.

So, we reduce the verbosity level to -d+1, which only gives the
Jam-level command. So now, it looks like:

common.copy /home/test/outputs/e/host/usr/aarch64-buildroot-linux-gnu/sysroot/usr/include/boost/geometry/multi/multi.hpp
common.mkdir /home/test/outputs/e/host/usr/aarch64-buildroot-linux-gnu/sysroot/usr/include/boost/geometry/strategies
gcc.compile.c++ bin.v2/libs/regex/build/gcc-4.7.3/release/threading-multi/icu.o
gcc.compile.c++ bin.v2/libs/regex/build/gcc-4.7.3/release/threading-multi/regex_debug.o
gcc.compile.c++ bin.v2/libs/regex/build/gcc-4.7.3/release/threading-multi/regex_raw_buffer.o

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>