Commit 32aaaaa led to failures when openwrt ARCH did not match kernel
ARCH, and this may not be its only side-effect.

This restores the previous Build/Compile and Build/Install, using the
default ones only when using external toolchain; in this case, ARCH is
set to LINUX_KARCH.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>