Restore audioio in Espressif for boards that can use it

[dhalbert]

Looks like audioio got disabled inadvertently due to some edits in mpconfigboard.mk. There was a change in from CIRCUITPY_AUDIOIO = 1 to CIRCUITPY_AUDIOIO ?= 1, and that appears not to be overriding the default CIRCUITPY_AUDIOIO ?= 0 earlier in the file.

[dhalbert]

A second ?= in a makefile for the same variable is ignored, per https://www.gnu.org/software/make/manual/html_node/Setting.html

If you’d like a variable to be set to a value only if it’s not already set, then you can use the shorthand operator ‘?=’ instead of ‘=’. These two settings of the variable ‘FOO’ are identical (see The origin Function):

FOO ?= bar

and

ifeq ($(origin FOO), undefined)
FOO = bar
endif

Example:
Makefile:

CIRCUITPY_AUDIOIO ?= 0
CIRCUITPY_AUDIOIO ?= 1

print-%:
        @echo "$* = "$($*)

% make print-CIRCUITPY_AUDIOIO
CIRCUITPY_AUDIOIO = 0