[Buildroot] Build regression after 2022.02.01

[Kristof Havasi]

Hi there!

I just updated from 2022.02 to 2022.02.01.

My build broke due to 8945ba49480a7885d10444
"toolchain/toolchain-external: error if BR2_TOOLCHAIN_EXTERNAL_PATH is not set"

I selected a bootlin toolchain for armv7, which is installed on the
build system and is available
via PATH. As the help string of BR2_TOOLCHAIN_EXTERNAL_PATH describes, if it is
empty, then PATH will be searched.

If I revert the above commit the build succeeds like before.

I could see the reason behind the check in the commit for the case,
where the toolchain is yet to be downloaded, but then the lines ended
up in the wrong
branch of the if/else statement.

In case the toolchain is already available via PATH, I suggest to
follow the previous logic
and description in the help text:
     "If empty, the compiler will be searched in $PATH."

Best Regards,
Kristóf