bootstrap.sh: use git if user wants to

If an argument is given to bootstrap.sh, it is used as a git commit
hash. Buildroot is git cloned and the tree is reset to the given commit.
Fix custom paths

Since buildroot is now in the directory "buildroot" we must add "../"
before all custom paths.
Better menu organization for custom projects
Fix projects.mk example
bootstrap.sh: use git am for git patches

All patches in "custom/patchs/git" directory will be applied with git am
in the buildroot directory.
Add a simple Makefile

This makefile only call buildroot Makefile with user's arguments.