toolchain: add missing stamp file touch in external toolchain

In the support of custom external toolchains, we forgot to touch the
$(STAMP_DIR)/ext-toolchain-checked stamp file, which means that the
toolchain was re-checked and re-installed at everyt build.

At the same time, quietize a bit other stamp files touch.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>