Fix nondeterministic load of fdt_slicer

[bdragon]

Summary

In r347183, the initialization of geom was moved from SI_SUB_DRIVERS SI_ORDER_FIRST to SI_ORDER_SECOND. Since fdt_slicer depends on initializing AFTER geom (due to being an extension of g_flashmap), it needs to be bumped to SI_ORDER_THIRD to ensure both geom and g_flashmap are set up before initializing.

The mmcsd module (which is the other module extending g_flashmap) uses the defaults and shouldn't be affected by this issue.

I encountered this issue when working on clang fixes for the QORIQ64 (Book-E powerpc64) kernel.

[royger]

LGTM, it's a shame we don't have a more declarative way to express such dependencies.

[jhibbits]

@kevans committed this in r352863 2 weeks ago.