packages: refactor checks using BR_BUILDING

Instead of manually testing MAKECMDGOALS, use the newly introduced
BR_BUILDING variable to know if we're building or not.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>