Fix selection of bootloaders from $(BR2_EXTERNAL)

If a package is based on "generic-package", pkg-generic.mk will compute
the name of the Kconfig variable to use for checking if this package has
been selected by the user.

Unfortunately, this mechanism does not take into account the case where
a bootloader is declared in a $(BR2_EXTERNAL)/boot directory.

So, even if the bootloader has been selected, it will not be added to
$(TARGETS) and will not be built.

This patch fixes this issue.

[Arnout: use single filter line, also apply this to toolchain]

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>