readme.md: remove "IDF" from Espressif reference

This change is to avoid misunderstanding as IDF is not supported
on top of MCUboot.
The Espressif port acts as a bare metal bootloader that can be
used on Espressif chips to boot supported OSes (like Zephyr or NuttX).

Signed-off-by: Almir Okato <almir.okato@espressif.com>