buildroot

genrandconfig: fix code style

Fix these warnings:
E201 whitespace after '['
E202 whitespace before ']'
E501 line too long (138 > 132 characters)
 -> isolate the common part of the external toolchain url in a variable
 to make the long lines shorter and more readable.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>