toolchain-external: make preinstall the default for custom toolchains

All the defconfig files used by the autobuilders that use
pre-installed external toolchains are making the assumption that the
default for a custom external toolchain is "pre-installed". We keep
this default for now, since changing it breaks the autobuilders.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>