openblas: fix config breakage introduced for configs using 'false'

My earlier change mistakenly expected `toString false` to produce '0'
instead of the empty string, leading to unexpected config changes.

Intended to address issue mentioned here and in following discussion:

https://github.com/NixOS/nixpkgs/pull/53972#issuecomment-459981602

Sorry, folks!

(special-case handling of bools here makes this "cleanup" a bit
 less of an obvious win but hopefully still preferable overall :))

-----------

makeFlags in resulting derivation, according to this one-liner:

$ nix show-derivation -f . openblas|jq ".[].env.makeFlags"

before:
"BINARY=64 CC=cc CROSS= DYNAMIC_ARCH=1 FC=gfortran HOSTCC=cc INTERFACE64=1 NO_BINARY_MODE= NO_STATIC=1 NUM_THREADS=64 PREFIX=/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9 TARGET=ATHLON USE_OPENMP=1"

after:
"BINARY=64 CC=cc CROSS=0 DYNAMIC_ARCH=1 FC=gfortran HOSTCC=cc INTERFACE64=1 NO_BINARY_MODE=0 NO_STATIC=1 NUM_THREADS=64 PREFIX=/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9 TARGET=ATHLON USE_OPENMP=1"

Without knowing how `placeholder` works, it seems interesting if
entirely unrelated that the `PREFIX` is same for both! :). TIL.