LU-19053 build: allow specifying "make rpms" build dir

LU-19053 build: allow specifying "make rpms" build dir

Currently "make rpms" will create a temporary directory with mktemp
to hold the intermediate build products, and this ends up in /tmp.
This can cause issues if /tmp is not large enough for the full build.

Allow specifying "BUILDDIR=DIR" to redirect the intermediate build
products into the specified directory. This allows you to run:

    BUILDDIR=/var/tmp make rpms

or

    BUILDDIR=/var/tmp make debs

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I12f2e7444f0fc7f09f41d64b8e4dd4a429797a37
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59410
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>