build: use build/$(mode).$(arch) in preference to the build/$(mode) symlink

The symlink is ambiguous when working with multiple targets, so avoid it,
esp. when embedding the output path in the build directory Makefile.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: avoid referring to $(out) in internal Makefile

We're already building in the output directory, no need to state it
explicitly.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: rename *.mak to *.mk

For consistency, and because eclipse groks .mk better.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: move mode-related configuration to conf/*.mk

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
build: move arch-specific configuration to conf/$(arch).mk

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Merge branch 'build'

Some build cleanups.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>