[TW#26745] ESPPORT not being forwarded properly

[sysprogs]

Currently CMake-based ESP-IDF does not properly handle the ESPPORT environment variable.
It happens due to the following line in run_esptool.cmake:

if(NOT ENV{ESPPORT})
    message("Note: esptool.py will search for a serial port. To specify a port, set the ESPPORT environment variable.")
else()
    set(port_arg "-p $ENV{ESPPORT}")
endif()

As something like "COM1" is not a valid boolean expression from CMake's point of view, the "NOT" branch is taken even if the environment variable is set.

You can fix it by using the following check instead:
if("$ENV{ESPPORT}" STREQUAL "")