These are core configurations providing PYBv1.x-level features.

This patch forces a board to explicitly define TEXT1_ADDR in order to
split the firmware into two separate pieces.  Otherwise the default is now
to produce only a single continuous firmware image with all ISR, text and
data together.

This patch allows a particular board to independently specify the linker
scripts for 1) the MCU memory layout; 2) how the different firmware
sections are arranged in memory.  Right now all boards follow the same
layout with two separate firmware section, one for the ISR and one for the
text and data.  This leaves room for storage (filesystem data) to live
between the firmware sections.

The idea with this patch is to accommodate boards that don't have internal
flash storage and only need to have one continuous firmware section.  Thus
the common.ld script is renamed to common_ifs.ld to make explicit that it
is used for cases where the board has internal flash storage.

This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.

The name used in py/mpconfig.h is MICROPY_FLOAT_IMPL so rename this
Makefile variable to mirror that.