toolchain-external: trivial clean up of messages

Before this commit, the output of the toolchain-external build steps
looked like this (abbreviated for clarity):

>>> toolchain-external undefined Building
>>> toolchain-external undefined Installing to staging directory
>>> toolchain-external undefined Copying external toolchain sysroot to staging...
>>> toolchain-external undefined Building ext-toolchain wrapper
mkdir -p output/host/usr/bin; cd output/host/usr/bin; for i in ...
/usr/bin/gcc -O2 -Ioutput/host/usr/include -DBR_SYSROOT='...
if test -f output/host/usr/bin/i686-pc-linux-gnu-gdb ; then mkdir -p ...
>>> toolchain-external undefined Fixing libtool files
>>> toolchain-external undefined Installing to target
>>> toolchain-external undefined Copying external toolchain libraries to target...
if test -e output/target/lib/ld-uClibc.so.1; then ln -sf ld-uClibc.so.1 output/target/lib/ld-uClibc.so.0 ; fi
if test -e output/target/lib/ld64-uClibc.so.1; then ln -sf ld64-uClibc.so.1 output/target/lib/ld64-uClibc.so.0 ; fi

All the long lines with conditions and loops in them are not usefull,
so put $(Q) in front of them. The line with mkdir can better be split
on a separate line so the cd stands out more. There are two redundant
semicolons that can be removed. The installation of gdbinit could
use an extra message so the user can see what is going on.

After this commit, the toolchain-external build steps look like this:
>>> toolchain-external undefined Building
>>> toolchain-external undefined Installing to staging directory
>>> toolchain-external undefined Copying external toolchain sysroot to staging...
>>> toolchain-external undefined Building ext-toolchain wrapper
/usr/bin/gcc -O2 -Ioutput/host/usr/include -DBR_SYSROOT='...
>>> toolchain-external undefined Installing gdbinit
>>> toolchain-external undefined Fixing libtool files
>>> toolchain-external undefined Installing to target
>>> toolchain-external undefined Copying external toolchain libraries to target...

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>