populate_sdk_base/image: Fix races for variable mappings

populate_sdk_base/image: Fix races for variable mappings

When using the -c populate_sdk option, images are not generated quite as
they should be under certain circumstances. For example the dropbear
feature may not get replaced with openssh, leading to both being installed
with an appropriate rootfs failure.

This patch moves the remapping logic to later points in the code, ensuring
there is no conflict. The result is slightly simpler too as an added bonus.

[YOCTO #3749]

(From OE-Core rev: 90cfa16bd4a02ada84ef94f6ae6f182beb8bdc01)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>