bootchart2: Fix usrmerge support

bootchart2: Fix usrmerge support

bootchart2 introduced a variable EARLY_PREFIX for supporting systems
with usrmerge [1].  Right now, the recipe here is sidestepping this
feature and trying to replicate it by overwriting other variables and
even patching the sources.  This wasn't enough, however, as there are
still problems:  For example, some setup code in the bootchart-collector
fails because it expects EARLY_PREFIX to be used [2].

Cleanup the recipe to set EARLY_PREFIX and remove the other workarounds.

[1]: https://github.com/xrmx/bootchart/commit/56a638ace1d172163b6d636c89892446b8add4b6
[2]: https://github.com/xrmx/bootchart/blob/3d2136d0335718fbe1a8e2370ccbc30123a6e593/collector/collector.c#L670-L672

Fixes: 4157600d3122 ("bootchart2: switch to add patch from change source in do_install")
Signed-off-by: Harald Seiler <hws@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>