build: Allow building only the support libraries

When using a build server it is desirable to build all the supported
targets. However, if the support libraries can't be built, it doesn't
make sense to still try to build each target.

This rework the top makefile to only build the support libraries when
no TARGET is given.

Change-Id: Ie75fa06580304a48fb33d373fe9cee5f75244c64
Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Reviewed-on: http://review.adnet.avionic-design.de/1239
Reviewed-by: Dirk Leber <dirk.leber@avionic-design.de>
build: Move the build directory to $(prefix)/build

Once mpfr has been built it is not possible to change the prefix
anymore. This prevent using the same source tree to build for several
prefixes, for example for test builds. As a workaround move the build
dir to the install directory.

Change-Id: Ie4a230528861fe77ef9864fce9dd6d9138591158
Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Reviewed-on: http://review.adnet.avionic-design.de/1240
Reviewed-by: Hendrik Lippek <hendrik.lippek@avionic-design.de>
build: Drop the rules for the non-existing newlib

This library has never been in stage1, so drop the rules referencing
it.

Change-Id: I02f07d7baafc0ec9b5b495964da344362e78bff1
Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Reviewed-on: http://review.adnet.avionic-design.de/1242
Reviewed-by: Dirk Leber <dirk.leber@avionic-design.de>