build: Drop stray echo command in llvm-configure target

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Drop --enable-assertions from configure runs where it is not supported
build: Drop unnecessary header copies from stp-configure commands

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Drop redundant cp commands in stp-make recipes

This is already taken care of by the stp-copy targets.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Use CP variable in all cases

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Replace shell invocation by $(wildcard) Make function

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Run stp configure script with /bin/sh instead of bash.

There is nothing bash-specific in that script.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
qemu/configure: Allow passing extra CXXFLAGS
docs: small typo fixes