classes: bundle: use getVarFlag instead of .get

[js731ca]

Make use of .getVarFlag instead of the simple .get to allow bitbake level variable expansion.

This is useful especially when using https://rauc.readthedocs.io/en/latest/advanced.html#sec-variants with the device-tree compatible constructed from another variable in e.g. the machine config

for consistencies sake the other/close slotfllag.get calls where also replaced

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Example, from a bundle recipe:

RAUC_SLOT_bootloader = "barebox-foo"
RAUC_SLOT_bootloader[name] = "bootloader.foo,product-Rev${MACHINE_REVISION}"                                      
RAUC_SLOT_bootloader[type] = "boot"
...

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Sidenote: i currently don't have a full build+config setup for something newer than kirkstone... but the changes should port easily?

[jluebbe]

This part has been changed significantly by #289 and #336 since kirkstone. Variable expansion should work there.

@ejoerns Should those changes be backported to kirkstone or would you take this change?

[ejoerns]

@js731ca Thank your for your fix. As @jluebbe already mentioned, we have existing fixes for this in master/scarthgap.

I just checked that they are backportable with manageable effort and seem to work well under kirkstone, too.

I prefer the backport way since it ensures that master and kirkstone version of bundle.bbclass stay more similar which should ease further backports.