support/testing/infra/builder: configure and build with make target and environment

Make the builder able to call 'VAR1=1 make VAR2=2 target'.

Allow sending extra parameters to be added to the end of make command
line. Uses for these purposes:
 - to configure a br2-external, using the 'BR2_EXTERNAL="dir" variable.
 - to specify a make target, such as 'foo-source.'

Allow adding variables to the environment when calling make.
These added variables allow a user to override default values from BuildRoot,
such as 'BR2_DL_DIR="dl"'.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Daniel J. Leach <dleach@belcan.com>
Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>