package/ncurses: Remove duplicated library install code

Currently, the ncurses package conditionally defines macros to install
the panel, form and menu libraries. This means we duplicate the install
step for each library type.

Rather than defining a set of macros, this change introduces a variable
for the set of installed libraries, $(NCURSES_LIBS-y). We use this in a
single macro to perform the library installation in one place.

Based on a change suggested by Thomas Petazzoni
<thomas.petazzoni@free-electrons.com>.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>