cmake: esp32: use zephyr defined python tool

This configures esptool, partition generator and
external bootloader build to use python tool defined
in Zephyr's context environment. This fixes error
regarding missing /usr/bin/python when python2.7 is not
installed in users's machine.

Partition table offset is also updated as part of
this change.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>