Use syscfg settings for sysinit stage numbers

[supervillain101]

This allows the app or target to rearrange package initialziation order via syscfg overrides

[sjanc]

lets wait with this until conclusion on similar thing in core is arranged https://github.com/apache/mynewt-core/pull/2972

[supervillain101]

@sjanc https://github.com/apache/mynewt-core/pull/2972 was finally merged in, can we do this one as well?

[sjanc]

please rebase (ie no merge master commits)

[supervillain101]

@sjanc I have rebased. Thanks!