enable serial console on fam15h boards

config/coreboot/kcma-d8-rdimm_16mb/config/libgfxinit_txtmode

config/coreboot/kcma-d8-rdimm_2mb/config/libgfxinit_txtmode

config/coreboot/kcma-d8-udimm_16mb/config/libgfxinit_txtmode

config/coreboot/kcma-d8-udimm_2mb/config/libgfxinit_txtmode

config/coreboot/kfsn4-dre_1mb/config/libgfxinit_txtmode

config/coreboot/kfsn4-dre_2mb/config/libgfxinit_txtmode

...ig/coreboot/kgpe-d16-rdimm_16mb/config/libgfxinit_txtmode

config/coreboot/kgpe-d16-rdimm_2mb/config/libgfxinit_txtmode

...ig/coreboot/kgpe-d16-udimm_16mb/config/libgfxinit_txtmode

config/coreboot/kgpe-d16-udimm_2mb/config/libgfxinit_txtmode