build: skip toolchain detection for some targets

Most developers run the `clean`, `checkpatch` and other similar targets
without specifying any other additional build options. When combined
with a flow where the developer passes `CROSS_COMPILE` or `CC`
explicitly, and where the default-configured tools are not on the PATH,
these targets will warn about unrecognized toolchain tools.

This change is a workaround for this whereby the toolchain makefile is
not expanded unless a target *not* named `*clean`, `check*` `doc` or
`*tool` has been specified.

Change-Id: I2f2a275964b65253df07c2207043217b14f615fe
Signed-off-by: Chris Kay <chris.kay@arm.com>
4 files changed