kconfig: Use 'default' instead of 'def_bool' in Kconfig.defconfig files

Same deal as in commit 46386522 ("Kconfig: Use 'default' instead of
'def_bool' in Kconfig.defconfig files"), fixing new stuff that got
introduced since then.

Some symbols, like ALTERA_AVALON_PIO, are only defined in
Kconfig.defconfig files, and so need the def_bool.

Motivation (from the note at the end of
guides/kconfig/index.html#common-shorthands):

For a symbol defined in multiple locations (e.g., in a Kconfig.defconfig
file in Zephyr), it is best to only give the symbol type for the "base"
definition of the symbol, and to use 'default' (instead of 'def_<type>'
value) for the remaining definitions. That way, if the base definition
of the symbol is removed, the symbol ends up without a type, which
generates a warning that points to the other definitions. That makes the
extra definitions easier to discover and remove.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>