package/binutils: don't override the build command

package/binutils: don't override the build command

In 1d42d0acca (binutils: ensure TARGET_CONFIGURE_ARGS is taken into
consideration for subdirs), the whole BUILD_CMDS was overriden in an
attempt to ensure that the target configure args (in fact, environment
variables) are indeed passed in the environment of the build command.

However, there is no reason to override the whole command, when we can
simply specify additional environment variables, as supported by the
autotools infra.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>