pbl multiimage: Allow to check image sizes ...

PBL images are often constrained in size by limitations exposed by
the SoCs SRAM size or partition sizes on the boot device. So far
we tried to configure these limits in Kconfig, but with PBL multi
images and thus different limitations for the different supported
images this no longer works. This patch has another approach for
it:

During build time make variables containing the relevant sizes for
each image are created. These are:

PBL_CODE_SIZE_$(symbol)
PBL_MEMORY_SIZE_$(symbol)
PBL_IMAGE_SIZE_$(symbol)

PBL_CODE_SIZE_$(symbol) contains the pure code size of the PBL, it
should be smaller than the available SRAM during boot. Normally the
PBL's bss segment also needs to be in the initial SRAM, for this
case PBL_MEMORY_SIZE_$(symbol) is the relevant variable.
PBL_IMAGE_SIZE_$(symbol) contains the full size of the PBL image
including the compressed payload (but without any image headers
created later by SoC specific image tools).

$(symbol) is a placeholder for the start symbol used for this PBL image,
thus for the i.MX53 QSB with entry start_imx53_loco

PBL_CODE_SIZE_start_imx53_loco

will be created. The images/Makefile.* can use these variables directly
to check sizes or specify the same variables with a "MAX_" prefix. So
when images/Makefile.imx specifies

MAX_PBL_CODE_SIZE_start_imx53_loco = 0x10000

then the build system will make sure that the PBL code for the QSB will
not get bigger than 64KiB.

Also included in this patch are the size restrictions for the i.MX8MQ
images as an example how to use this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>