package/rauc: select fw_printenv for uboot based systems

When operating on a uboot based system, rauc interacts with
the bootloader environment using fw_printenv and fw_setenv [1].

These commands should therefore be present on the target if
the system being built uses uboot.

[1] See:
https://github.com/rauc/rauc/blob/v1.2/src/bootchooser.c#L21-L22
https://github.com/rauc/rauc/blob/v1.2/src/bootchooser.c#L644-L645



Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>