should IMU frame follow the red-East, green-North and blue-Up convention?

edit

-Green axis points front(North) of vehicle -Red axis points right(East) of vehicle -Blue axis points up

No, "North" and "East" really mean "North" and "East" relative to the earth, not "front of vehicle" and "right of vehicle". The ENU convention means the following: If the device reports a neutral orientation (roll / pitch / yaw are all 0 in the sensor_msgs/Imu message), then the x axis (red) points East (relative to the earth), y (green) points North, and z points Up.

Edited to add: I just saw that you wrote that your IMU doesn't have absolute yaw. In that case, the x and y axis can point more or less into an arbitrary direction with respect to the earth (since the "zero yaw" position is also arbitrary with respect to the earth). To conform to the ENU convention, all that matters is that z points up. And of course the other fields in the sensor_msgs/Imu message are specified wrt. the coordinate axes; so x must point into the direction where linear_acceleration.x is positive and so on.

In any case, the ENU convention says nothing about how the IMU must be mounted on the vehicle.