Use AM_COND_IF() macro where possible.

Conditional actions in configure for which AM_CONDITIONAL has already
been set up may be tested again using AM_COND_IF, so replace the
custom AS_IF constructs to make it more readable.
Simplify libcintelhex linker flag handling.

Use AC_SEARCH_LIBS in place of AC_CHECK_LIB macro for the benefit of
automatically adding the needed -l flag to the @LIBS@ substitution.

For the case of using the embedded libcintelhex copy, list the static
library file directly instead of providing an -lcintelhex flag.
Remove the corresponding manual LDFLAGS and LDADD computations.
Adopt standard macro order in configure.ac.

Slightly shuffle around the typedef and library function checks,
conforming to the officially proposed configure.ac layout.  Invoke
AC_CONFIG_MACRO_DIR explicitly.
Mark find_string.c for translation.

The output message of the nvm_string_list() function should be
localized, so mark for translation.