Improve TARGETS handling for bootloaders and kernel

As the kernel and bootloaders do not use the normal BR2_PACKAGE_*
Kconfig options, their target name was not automatically added to the
global TARGETS variable. Each bootloader .mk and the linux.mk had to
add their own target manually to TARGETS, and the package
infrastructure was making tests on non-existing Kconfig variables.

This commit improves the package infrastructure so that it looks at
BR2_PACKAGE_<pkg> for packages, BR2_TARGET_<pkg> for bootloaders and
at the special BR2_LINUX_KERNEL for the linux package.

This allows to simplify a little bit the bootloaders and linux .mk
files.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>