bc: patch makeinfo logic

It uses a hardcoded "makeinfo" command invocation instead of the usual
MAKEINFO variable with the missing logic for docs. Fixes:
http://autobuild.buildroot.net/results/b0b/b0b9dced0014a5a8026fa972f95085e8bd16a07b/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>