toolchain-external: strip trailing slash from autogenerated FOO_SITE

Trailing slashes are going to be declared illegal from FOO_SITE
variables.

But Buildroot internally generates such a variable when using a custom
external toolchain (i.e. BR2_TOOLCHAIN_EXTERNAL_CUSTOM). This is
because TOOLCHAIN_EXTERNAL_SITE is set to
$(dir $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_URL))), and $(dir)
leaves a trailing slash.

Fix it using patsubst, just like linux and the bootloaders do.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>