bc: use u-a for bc as well

* bc can be provided by busybox as well (e.g. if you have your own
  defconfig and forget to explicitly disable it:
  ...
  *
  * Miscellaneous Utilities
  *
  adjtimex (4.7 kb) (ADJTIMEX) [N/y/?] n
  bbconfig (9.7 kb) (BBCONFIG) [N/y/?] n
  bc (45 kb) (BC) [Y/n/?] (NEW) dc (36 kb) (DC) [Y/n/?] y
    Use bc code base for dc (larger, more features) (FEATURE_DC_BIG) [Y] (NEW) y
      Interactive mode (+4kb) (FEATURE_BC_INTERACTIVE) [Y/n/?] (NEW)     Enable bc/dc long options (FEATURE_BC_LONG_OPTIONS) [Y/n] (NEW) beep (2.4 kb) (BEEP) [N/y/?] n
  chat (6.3 kb) (CHAT) [N/y/?] n
  conspy (10 kb) (CONSPY) [N/y/?] n
  ...
  ), causing conflict in u-a:

  update-alternatives: Error: not linking /usr/bin/bc to /bin/busybox.nosuid since /usr/bin/bc exists and is not a link

  and then whole do_rootfs or do_populate_sdk to fail because busybox postinst is failing:

  do_populate_sdk: Postinstall scriptlets of ['busybox'] have failed. If the intention is to defer them to first boot,
  then please place them into pkg_postinst_ontarget_${PN} (). Deferring to first boot via 'exit 1' is no longer supported.

(From OE-Core rev: f4e712162566373279049fb952d8bceb9184128a)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/bc/bc_1.07.1.bb

@@ -27,7 +27,7 @@ do_compile_prepend() {
     cp -f ${WORKDIR}/libmath.h ${B}/bc/libmath.h
 }
 
-ALTERNATIVE_${PN} = "dc"
+ALTERNATIVE_${PN} = "bc dc"
 ALTERNATIVE_PRIORITY = "100"
 
-BBCLASSEXTEND = "native"
+BBCLASSEXTEND = "native"