scripts: feeds: Don’t hardcode IS_TTY

scripts: feeds: Don’t hardcode IS_TTY

When building in environments that set IS_TTY, the feeds script does not
honor it and passes a hardcoded value to scan.mk, causing unwanted
control characters to appear in stdout.

This commit addresses the issue by checking IS_TTY and MAKE_TERMOUT
variables and uses their values if defined.

Closes #8039

Signed-off-by: Ernestas Kulik <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20743
Signed-off-by: Christian Marangi <[email protected]>
(cherry picked from commit fd7e43d429a370665795fb14c859c046c761e4e9)