greybus: firmware: replace colons with underscore in firmware file request

Due to some issues with handling colons in Android (and possibly
future exposure to other problems) we should remove the colons from
the firmware file request.  Replacing them with underscores.

Specifically, we copy firmware into the Android build using a line
similar to this:
PRODUCT_COPY_FILES += \
    source-repo/ara:00000126:00001000:00000001:00000001:02.tftf:system/etc/firmware/ara:00000126:00001000:00000001:00000001:02.tftf

There is a colon delimiter between the source and destination in the
PRODUCT_COPY_FILES format.  The greybus naming logic breaks the parsing
routine and generates an Android build break.

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>