Replace *MD5SUM with *HASH, replace MD5 hashes with SHA256

Signed-off-by: Felix Fietkau <nbd@nbd.name>

Add three download mirrors for 'bc', as the Makefile currently defines only
one download location.

@GNU can not be used, as the most recent version of 'bc' is not available at
the general GNU mirrors, and can only be found at the "gnu-alpha" mirrors.

Signed-off-by: Hannu Nyman <hannu.nyman at iki.fi>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 45908

Building the "bc" documentation requires the "makeinfo" program to be
installed on the host.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 37209

 - bc is required by the kernel to compute timeconsts files
 - fixes #13767.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>

SVN-Revision: 37197