support/kconfig/merge_config.sh: fix merging buildroot config files

support/kconfig/merge_config.sh: fix merging buildroot config files

This patch allows us to define config prefix with CONFIG_ environment
variable.

By setting the proper config prefix, we will have proper 'redundant
configuration warnings' when we use '-r -m' options.

This is actually already in mainline for v4.20-rc1:
2cd3faf87d2d8f6123adf34741b9a7b98828a76f
("merge_config.sh: Allow to define config prefix")

Signed-off-by: Nasser Afshin <afshin.nasser@gmail.com>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>