coreboot/dell: move e6400 to new tree, dell

the ddr2 fix broke *ddr3* on gm45 thinkpads in
testing, depending on memory modules. this was
established by removing patches, re-doing
configs etc, on a user's X200 (testing gentoo
and freebsd). the X200 kept randomly rebooting
or having random glitches.

the configs themselves (gm45 thinkpads) will
also be re-done, because i found minor issues
unrelated, but this patch moves dell e6400 to
its own tree. the ddr2 fix is no longer present
in coreboot/default, only coreboot/dell.

i noticed minor differences in gm45 thinkpad
configs, when re-doing the configs, versus
what are currently in lbmk master; for instance,
vbt was not enabled anymore, on thinkpad x200.
modifications to these will be done separately.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/dell/target.cfg

@@ -0,0 +1,4 @@
+tree="dell"
+romtype="normal"
+rev="d862695f5f432b5c78dada5f16c293a4c3f9fce6"
+arch="x86_64"