package/Makefile.in: update/fix HOST_PATH variable
package/pkg-cmake.mk: make sure $(HOST_PATH) is in the PATH at configure time
dependencies: build a host python2 if no suitable one can be found

Some distros now comes with only python3 installed, with or without a
/usr/bin/python pointing to python3.

So, /usr/bin/python may refer to either python2 or python3.

This may have some bad consequences for packages that uses some
non python{2,3}-friendly scripts in their build system (eg. in install
or post-install scripts).

This patch checks for a suitable python2 version (2.6 or 2.7) on the host
system, and:
* if a valid python2 interperter is found on the host system, then:
  - a symlink pointing to the found python2 interpreter is created in the
    buildroot host tree at the beginning of the build;
  - a make target host-python2-if-needed is set to '$(HOST_DIR)/usr/bin/python'.
    This way, if the host-python package is built later, the symlink will be
    automatically overriden by the host-python install rules.
* if no valid python2 interperter is found on the host system, then:
  - a make target host-python2-if-needed is set to 'host-python'.

Note 1: The symlink is checked and created if necessary at the beginning of
        each build.
Note 2: Since there is no target named '$(HOST_DIR)/usr/bin/python' in the
        python makefiles, there is no need for removing the symlink before
        installing the host-pyhton package.
Note 3: The host-python is only built when necessary.
        This also means that in the case there is no python2 interperter
        available on the host system, there won't any host python2 interperter
        until the host-python package is built.
Note 4: Any package containing some non python{2,3}-friendly scripts should:
        - have 'host-python2-if-needed' in its *_DEPENDENCIES list;
        - set and add to their build/install environment, either
        PATH="$(HOST_PATH)" or PATH="$(TARGET_PATH)".

A (good) side effect of this patch is getting rid of any host python dependency.

Buildroot can run on a host without python, or with a too old python2 version,
or with only python3.

In any case where a host python2 interpreter is expected to run some build or
install scripts, 'host-python2-if-needed' should be added to the dependency
list.

With this patch, the host python2 interpreter is either found in the host
system, or build by Buildroot. So, we can drop the python dependency check.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
scons: add host-python2-if-needed dependency

Also, ensure $(HOST_DIR)/usr/bin is in the PATH during the configure phase.
scons: ensure $(HOST_DIR)/usr/bin is in the PATH when invoking $(SCONS)
manual: add host python2 dependency section
python3: fix _decimal module build

Signed-off-by: Samuel Martin <s.martin49@gmail.com>