[sb2modes] Remove sb2 modes from sdk-sb2-config

Use scratchbox2's own obs-rpm-* modes by symlinking sdk-build,
sdk-build+pp and sdk-install correspondingly.

sdk-manage: cd to the target directory not needed when installing
packages to an sb2 target.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>