toolchain/toolchain-external: don't use x$(...) construct or ==

With modern shells, we can simply do test using the "$(...)" = "value"
form. This commit gets rid of the x$(...) = x"value" constructs and
replaces == by =, which is the correct operator to test the equality
of two strings with the test program.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>