Makefile.toolchain.xcc: support multiple builds

XTENSA_TOOLS_PATH and XTENSA_BUILDS_PATH have been retired.

XTENSA_SDK is now the base directory for the Xtensa SDK. The build
systems will search for toolchains in there, using
CONFIG_TOOLCHAIN_VARIANT to locate the right one. It defaults to
/opt/xtensa.

XTENSA_BUILD_PATHS is now a list of additional directories to search
for Xtensa CPU builds. By default the build system will already search
the builds included in the SDK; this is intended for vendor-supplied
CPU build definitions.

Some whitespace changes made for readability and comments added.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>