ALT_BOOTDIR problem for gentoo

[GNUtoo at no-log.org]

hello,
I am trying to make a gentoo ebuild for icedtea
An ebuild is a little bit like bash scripts for installing configurating
compiling and installing the program
The problem with the ebuilds is that the variables inside the makefile
doesn't get exported or unset...so you have to unset or export them in the
ebuild

so here's the problem...compiling it manualy following the howto for
gentoo works...
but within the ebuild i have a problem with the variable ALT_BOOTDIR
i've set the variable :
ALT_BOOTDIR="${WORKDIR}/${PN}/bootstrap/jdk1.6.0"
${WORKDIR}/${PN} are the locations of the sources

it says that:
RROR: Your BOOTDIR environment variable does not point
       to a valid JDK for bootstrapping this build.
       A JDK 7  build must be bootstrapped using
       JDK 1.5.0 fcs (or later).
       Apparently, your bootstrap JDK is version 1.4
       Please update your ALT_BOOTDIR setting and start your build

what should i do?
i do not undestand well the process of bootstraping...(the compilers it
uses and similar like gcc->gcj->ecj->icedtea)