jimtcl: fix installation in BR2_STATIC_LIBS case

As noticed by Yann E. Morin in the review of
http://patchwork.ozlabs.org/patch/429533/

, there was something fishy
in the jimtcl installation logic:

  ln -s libjim.$(JIMTCL_LIB) $(STAGING_DIR)/usr/lib/libjim.so

where JIMTCL_LIB has the value 'a' for BR2_STATIC_LIBS=y builds. Which
means we're linking libjim.so to libjim.a. Not great.

This commit therefore reworks the installation logic of the jimtcl.mk
package to install the shared library when BR2_STATIC_LIBS is not set,
and the static library when BR2_STATIC_LIBS is enabled. The macro
JIMTCL_INSTALL_LIB now takes as argument where the library should be
installed, so that it can be used for both the target and staging
installations.

Note that we can only either build the shared library *or* the static
library with the jimtcl build system. There is no possibility of
building both.

Reported-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>