scripts: add BUILD/HOST extra cflags/ldflags

On some hosts, and for certain toolchains (eg. toolchain targetting
the upcoming Darwin), it may be necessary to pass arbitrary CFLAGS
and/or LDFLAGS when building the components.

And necessary infrastructure:
- EXTRA_{CFLAGS,LDFLAGS}_FOR_{BUILD,HOST} as config options
- pass those extra flags to components

Fix-up a slight typo in elf2flt at the same time (misnamed cflags).

Signed-off-by: Yann Diorcet <diorcet.yann@gmail.com>
Message-Id: <d24043276c9243a35421.1353077450@macbook-smorlat.local>
Patchwork-Id: 199645

 # Compute microblaze specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     # CFLAGS

     case "${CT_ARCH_FLOAT_HW},${CT_ARCH_FLOAT_SW}" in

         y,) CT_ARCH_FLOAT_CFLAG="-mhard-float" ;;

         ,y) CT_ARCH_FLOAT_CFLAG="-msoft-float" ;;

     esac

 }