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).