exim: new package

[Thomas:
 - use $(INSTALL) instead of install
 - add AR and RANLIB variables in the local makefiles, so that the
   cross toolchain ar and ranlib utilities are used instead of the
   native ones.
 - move the init script initialization to the EXIM_INSTALL_INIT_SYSV
   variable.
 - Use parenthesis instead of curly braces to reference TARGET_DIR.]

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>