Makefile: move some variable definitions to the top Makefile

This commit moves some variable definitions from config.mk
to the top Makefile:

  - HOSTCC, HOSTCFLAGS, HOSTLDFLAGS
  - AS, LD, CC, CPP, etc.
  - SHELL (renamed to CONFIG_SHELL)

I'd like to slim down config.mk file
because it is included from all recursive make.
It is redundant to re-define the variables
every time descending into sub directories.
We should rather define them at the top Makefile
and export them.

U-Boot makefiles has been used "SHELL" variable to store shell
chosen for the user, whereas Linux Kernel uses "CONFIG_SHELL".

We should never use "SHELL" variable because it is
a special variable for GNU Make.
Changing SHELL may cause unpredictable side effects
whose root cause is usually difficult to find.
We should use a generic variable name "CONFIG_SHELL".

We should not use the syntax as follows either:

    rm -f $(obj)tools/env/{fw_printenv,fw_setenv}

This depends on "bash" although GNU Make generally
invokes "sh" to run the each rule.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>