introduce new feature: BOOTLOADER_IGNOREPROGBUTTON

"CONFIG_HAVE__BOOTLOADER_IGNOREPROGBUTTON" generates an USBaspLoader
without using the PROGBUTTON.
It can be used to reduce the required PINcount for USBaspLoader on the MCU.

However this feature is very dangerous, so it becomes only enabled, if
"CONFIG_HAVE__BOOTLOADER_ALWAYSENTERPROGRAMMODE" is enabled and
"CONFIG_NO__BOOTLOADER_CAN_EXIT" is disabled, too.

Additionally "BOOTLOADER_LOOPCYCLES_TIMEOUT" must be greater or equal
than 8 (In order to give user enough time to program).

When active, "JUMPER_PORT" and "JUMPER_BIT" are ignored and can be soldered
otherwise.

Default if off. But the Option can be enabled by adding
"-DCONFIG_HAVE__BOOTLOADER_IGNOREPROGBUTTON" to the DEFINES variable of
Makefile.inc

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>